AGESA boards: Clean up definition of BIOS_SIZE in platform_cfg

Clean up vendor code from hard coded #define if-def chain with a
pre-processor shift and subtract.

Change-Id: Ibce34ab576d7db8586a6ec8f9b2460268e0e1878
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/4811
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Tested-by: build bot (Jenkins)
12 files changed