arch/x86: unexpose postcar_frame_common_mtrrs()

The only caller is contained within the postcar_loader compilation unit.
Therefore, remove postcar_frame_common_mtrrs() from the global symbol
namespace.

Change-Id: I90d308669d13eb2bebf1eca4d47e3f3b4f178714
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41101
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>

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

@@ -41,11 +41,6 @@ void postcar_frame_add_mtrr(struct postcar_frame *pcf,
  */
 void postcar_frame_add_romcache(struct postcar_frame *pcf, int type);
 
-/*
- * Add a common MTRR setup most platforms will have as a subset.
- */
-void postcar_frame_common_mtrrs(struct postcar_frame *pcf);
-
 /*
  * fill_postcar_frame() is called after raminit completes and right before
  * calling run_postcar_phase(). Implementation should call postcar_frame_add_mtrr()

src/arch/x86/postcar_loader.c

@@ -111,7 +111,7 @@ void postcar_frame_add_romcache(struct postcar_frame *pcf, int type)
 	postcar_frame_add_mtrr(pcf, CACHE_ROM_BASE, CACHE_ROM_SIZE, type);
 }
 
-void postcar_frame_common_mtrrs(struct postcar_frame *pcf)
+static void postcar_frame_common_mtrrs(struct postcar_frame *pcf)
 {
 	if (pcf->skip_common_mtrr)
 		return;