Hook up MRC cache update

Requirements:
  - must be in ramstage (locking flash while executing code from there
    might not work)
  - must be after cbmem is reinitialized (so the mrc cache copy of the
    current run can be found)

Change-Id: I8028fb073349ce2b027ef5f8397dc1a1b8b31c02
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/1002
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/include/cbmem.h

@@ -73,6 +73,7 @@ extern struct cbmem_entry *get_cbmem_toc(void);
 
 #ifndef __PRE_RAM__
 void set_cbmem_toc(struct cbmem_entry *);
+void __attribute__((weak)) cbmem_post_handling(void);
 #endif
 #endif
 #endif