southbridge/bd82x6x: Reserve 16 MiB for flash and not 8.

X230 has 12 MiB flash. SPI controller supports up to 2 x 16 MiB of flash
but address map limits this to 16MiB.

Change-Id: Icc39c3c8d45d2d14e437bdfce920f8b4b039789d
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/5133
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>