soc/intel/common: sanity check ebda signature

It's possible for chipsets utilizing ebda to cache the cbmem_top()
value to be called prior to the object being entirely setup. As
such it's important to check the signature to ensure the object
has been initialized. Do that in a newly introduced function,
retrieve_ebda_object(), which will zero out the object if the
signature doesn't match.

Change-Id: I66b07c36f46ed9112bc39b44914c860ba68677ae
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/21995
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Subrata Banik <subrata.banik@intel.com>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>

src/soc/intel/skylake/memmap.c

@@ -348,7 +348,7 @@ void *cbmem_top(void)
 	if (sa_get_tseg_base() == 0)
 		return NULL;
 
-	read_ebda_data(cfg, sizeof(*cfg));
+	retrieve_ebda_object(cfg);
 
 	return (void *)(uintptr_t)cfg->tolum_base;
 }