include: Make DRAM an explicit region

This patch makes DRAM an explicit region by introducing
DECLARE_OPTIONAL_REGION(dram) and DRAM_END().

Note: many SoC platforms determine DRAM size and layout dynamically
during boot, making a static compile-time value is not feasible always.
Attempting to use REGION_SIZE(dram) in this scenario would result in a
missing symbol `_dram_size` error.

By making dram an optional region, we allow its size and address to be
defined only when available, preventing build failures on platforms
that configure DRAM dynamically.

The old extern u8 _dram[] is removed, as it's now covered by the new
region definition.

This is a preparatory step for future changes that will make use of
the new DRAM_END() macro.

This symbol is necessary for systems that require
the DRAM size to be known and accessible from the linker script or
other parts of the build system.

Additionally, a new macro DRAM_END(addr) is defined in memlayout.h.
This macro provides a consistent way to mark the end of the DRAM
region, similar to how REGION_END and other start/end macros are used
throughout the codebase.

TEST=Able to build and boot google/quenbi.

Change-Id: Ib98ec4b991eed56385c83be6a9ca39ff1380ff1b
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/88751
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Kapil Porwal <kapilporwal@google.com>

src/include/memlayout.h

@@ -60,6 +60,8 @@
 #define SRAM_END(addr) REGION_END(sram, addr)
 
 #define DRAM_START(addr) REGION_START(dram, addr)
+/* This should rarely be used because DRAM-size is usually not compile-time constant. */
+#define DRAM_END(addr) REGION_END(dram, addr)
 
 #define TIMESTAMP(addr, size) \
 	REGION(timestamp, addr, size, 8) \

src/include/symbols.h

@@ -5,8 +5,6 @@
 
 #include <types.h>
 
-extern u8 _dram[];
-
 #define REGION_SIZE(name) ((size_t)_##name##_size)
 
 #define DECLARE_REGION(name)	\
@@ -28,6 +26,7 @@ extern u8 _dram[];
 	__maybe_unused __weak extern u8 _##name##_size[];
 
 DECLARE_REGION(sram)
+DECLARE_OPTIONAL_REGION(dram)
 DECLARE_OPTIONAL_REGION(timestamp)
 DECLARE_REGION(preram_cbmem_console)
 DECLARE_REGION(cbmem_init_hooks)