soc/intel/common: Add feature directory for SoC-specific common code

Introduce a new directory structure src/soc/intel/common/feature/ for
sharing SoC-specific code across Intel SoC generations to reduce code
duplication.

Unlike the common block code (src/soc/intel/common/block/) which is
intended for reusable IP blocks, the feature code is for SoC-specific
functionality that is similar (but not identical) across multiple
generations. Platform-specific differences are handled through
configuration options or platform-specific macros.

This commit:
- Creates src/soc/intel/common/feature/ directory
- Adds feature/Kconfig defining SOC_INTEL_COMMON_FEATURE
- Adds feature/Makefile.mk to build subdirectories
- Updates src/soc/intel/common/Kconfig.common to source feature/Kconfig
- Updates src/soc/intel/common/Makefile.mk to include feature/ subdirs
- Documents the common code directory structure in
  Documentation/soc/intel/code_development_model/code_development_model.md

Change-Id: Idb842376a0a785a6439eeeb5a3a934d0bc575b09
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/91360
Reviewed-by: Bora Guvendik <bora.guvendik@intel.corp-partner.google.com>
Reviewed-by: Jakub "Kuba" Czapiga <czapiga@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Pranava Y N <pranavayn@google.com>

Documentation/soc/intel/code_development_model/code_development_model.md

@@ -87,9 +87,13 @@ Code design after common code in coreboot will look as follows:
 
 [coreboot_common_code_design]: coreboot_common_code_design.png
 
-There will be still some duplicated files left in each SOC folder and we may
-copy across a SOC as a base but these files are subject to change as
-development continues.
+There will be still some duplicated files left in each SOC folder and we
+may copy across a SOC as a base but these files are subject to change as
+development continues. Some of those files, presenting strong similarities
+from generation to generation, can be consolidated into the common feature
+directory to reduce code duplication and improve
+maintenance. Platform-specific differences are handled through
+configuration options or platform-specific macros.
 
 ## Benefits
 

src/soc/intel/common/Kconfig.common

@@ -94,6 +94,9 @@ if SOC_INTEL_COMMON
 comment "Intel SoC Common Code for IP blocks"
 source "src/soc/intel/common/block/Kconfig"
 
+comment "Intel SoC Common Code for features"
+source "src/soc/intel/common/feature/Kconfig"
+
 comment "Intel SoC Common PCH Code"
 source "src/soc/intel/common/pch/Kconfig"
 

src/soc/intel/common/Makefile.mk

@@ -3,6 +3,7 @@ ifeq ($(CONFIG_SOC_INTEL_COMMON),y)
 
 subdirs-y += basecode/
 subdirs-y += block/
+subdirs-y += feature/
 subdirs-y += pch/
 
 verstage-$(CONFIG_SOC_INTEL_COMMON_RESET) += reset.c

src/soc/intel/common/feature/Kconfig

@@ -0,0 +1,18 @@
+## SPDX-License-Identifier: GPL-2.0-only
+
+config SOC_INTEL_COMMON_FEATURE
+	bool
+	help
+	  Intel SoC common feature code. This option enables sharing of
+	  SoC-specific code across Intel SoC generations to reduce code
+	  duplication. Unlike the common block code which is intended for
+	  reusable IP blocks, the feature code is for SoC-specific
+	  functionality that is similar across multiple generations but
+	  may have minor platform-specific differences handled through
+	  configuration options or platform-specific macros.
+
+if SOC_INTEL_COMMON_FEATURE
+
+source "src/soc/intel/common/feature/*/Kconfig"
+
+endif

src/soc/intel/common/feature/Makefile.mk

@@ -0,0 +1,6 @@
+## SPDX-License-Identifier: GPL-2.0-only
+ifeq ($(CONFIG_SOC_INTEL_COMMON_FEATURE),y)
+
+subdirs-y += ./*
+
+endif