cbmemc: Bump default to 128K

board_status shows that truncation of few KiB is pretty common.
So bump this value.

Change-Id: I78a16974846a59ee4eae782380e6d01d2fa324f2
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/6902
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>