arch/x86: Introduce postcar_frame_add_romcache()

Provide a common implementation to add an MTRR entry for memory-
mapped boot ROMs.

Change-Id: I9fabc6b87fb36dc3d970805eb804cd950b8849d4
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/26577
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/arch/x86/include/arch/cpu.h

@@ -282,6 +282,11 @@ void postcar_frame_init_lowmem(struct postcar_frame *pcf);
 void postcar_frame_add_mtrr(struct postcar_frame *pcf,
 				uintptr_t addr, size_t size, int type);
 
+/*
+ * Add variable MTRR covering the memory-mapped ROM with given MTRR type.
+ */
+void postcar_frame_add_romcache(struct postcar_frame *pcf, int type);
+
 /*
  * Push used MTRR and Max MTRRs on to the stack
  * and return pointer to stack top.

src/arch/x86/postcar_loader.c

@@ -113,6 +113,13 @@ void postcar_frame_add_mtrr(struct postcar_frame *pcf,
 	}
 }
 
+void postcar_frame_add_romcache(struct postcar_frame *pcf, int type)
+{
+	if (!IS_ENABLED(CONFIG_BOOT_DEVICE_MEMORY_MAPPED))
+		return;
+	postcar_frame_add_mtrr(pcf, CACHE_ROM_BASE, CACHE_ROM_SIZE, type);
+}
+
 void *postcar_commit_mtrrs(struct postcar_frame *pcf)
 {
 	/*