soc/intel/apollolake: Fix bitshift issue in bootblock

Fix issue where zero-sized BIOS region could cause bitshift
for '-1' which is an unspecified behavior.

Change-Id: Icb62bf413a1a0d293657503ef21fe97b5f9a5484
Signed-off-by: Andrey Petrov <andrey.petrov@intel.com>
Reviewed-on: https://review.coreboot.org/15727
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/intel/apollolake/bootblock/bootblock.c

@@ -98,7 +98,10 @@ static void cache_bios_region(void)
 		return;
 
 	/* Only the IFD BIOS region is memory mapped (at top of 4G) */
-	rom_size =  get_bios_size();
+	rom_size = get_bios_size();
+
+	if (!rom_size)
+		return;
 
 	/* Round to power of two */
 	alignment = 1 << (log2_ceil(rom_size));