arch/x86: Add pre- and post-memory platform hooks to romstage

Introduce platform_romstage_pre_mem() and platform_romstage_post_mem()
as weak symbols in the x86 romstage cycle.

These hooks allow SoCs and mainboards to execute low-level setup or
instrumentation immediately before and after memory initialization
without modifying the core romstage.c flow.

- platform_romstage_pre_mem: Called before mainboard_romstage_entry.
- platform_romstage_post_mem: Called after memory is up but while still
  running on the Cache-as-RAM (CAR) stack.

Change-Id: I59cb115de0d512106d9a029d683c10b025076893
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/90999
Reviewed-by: Jérémy Compostella <jeremy.compostella@intel.com>
Reviewed-by: Pranava Y N <pranavayn@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/x86/include/arch/romstage.h

@@ -7,6 +7,19 @@
 #include <stdint.h>
 #include <cpu/x86/mtrr.h>
 
+/*
+ * Platform-specific hooks for logic surrounding memory initialization.
+ *
+ * platform_romstage_pre_mem() is invoked before memory training.
+ *
+ * platform_romstage_post_mem() is invoked after DRAM is initialized but
+ * before the transition to the RAM-based stack (Post-CAR).
+ *
+ * Default: No-op. These are weak symbols to be overridden by SoC or mainboard.
+ */
+void platform_romstage_pre_mem(void);
+void platform_romstage_post_mem(void);
+
 void mainboard_romstage_entry(void);
 
 /*

src/cpu/intel/car/romstage.c

@@ -16,6 +16,19 @@
    following as a guideline for acceptable stack usage. */
 #define DCACHE_RAM_ROMSTAGE_STACK_SIZE 0x2000
 
+/*
+ * Platform-specific hooks for logic surrounding memory initialization.
+ *
+ * platform_romstage_pre_mem() is invoked before memory training.
+ *
+ * platform_romstage_post_mem() is invoked after DRAM is initialized but
+ * before the transition to the RAM-based stack (Post-CAR).
+ *
+ * Default: No-op. These are weak symbols to be overridden by SoC or mainboard.
+ */
+__weak void platform_romstage_pre_mem(void) { /* no-op */ }
+__weak void platform_romstage_post_mem(void) { /* no-op */ }
+
 void __noreturn romstage_main(void)
 {
 	int i;
@@ -71,7 +84,9 @@ void __noreturn romstage_main(void)
 	 */
 	romstage_adainit();
 
+	platform_romstage_pre_mem();
 	mainboard_romstage_entry();
+	platform_romstage_post_mem();
 
 	/* Check the stack. */
 	for (i = 0; i < num_guards; i++) {