arch/x86: cache postcar in stage cache

Stash and reload postcar stage in the stage cache for increased
S3 resume speed. It's impact is small (2 ms or so), but there's
no need to go to the boot media on resume to reload something
that was already loaded. This aligns with the same paths we take
on ramstage as well.

Change-Id: I4313794826120853163c7366e81346858747ed0a
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/17649
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/include/stage_cache.h

@@ -23,6 +23,7 @@
 enum {
 	STAGE_RAMSTAGE,
 	STAGE_REFCODE,
+	STAGE_POSTCAR,
 };
 
 /* Cache the loaded stage provided according to the parameters. */