kevin/custom.fmap: drop BIOS@0 region

On x86 platforms, it is necessary to wrap the entire image in a
BIOS@0 region which marks the memory-mapped flash image as needing
SMM protection.  Unfortunately this creates overlapping FMAP
regions, which flashrom cannot handle without a patch.

On arm64 platforms, the BIOS@0 region is not needed.  So let's not
include it.  This allows us to skip the flashrom patch on arm64.

src/platform/kevin/custom.fmap

@@ -10,7 +10,6 @@
 # +-------------+ <-- ROM_SIZE
 
 FLASH@0 0x1000000 {
-	BIOS@0 0x1000000 {
                 # read-only zone (eventually)
 		BOOTBLOCK                     128K
 		FMAP           @  0x20000    0x400
@@ -21,4 +20,3 @@ FLASH@0 0x1000000 {
 		NORMAL(CBFS)   @ 0x800000 0x7DF000
                 MISC           @ 0xFDF000  0x21000    # 132k
 }
-}