raspberrypi2: change GPU_MEM to 255

raspberrypi2: change GPU_MEM to 255

Scale back GPU to 256MB for successful booting on
Raspberry Pi 2. The same change has been already
applied to the configurations for Raspberry Pi 3
as part of the work on SPEC-410 and SPEC-469.

Bug-AGL: SPEC-616

Change-Id: Iafa83209d9e3b04609d820e3492ddb6266864f92
Signed-off-by: Leon Anavi <leon.anavi@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/9741
Reviewed-by: Thomas Rini <tom.rini@gmail.com>
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Reviewed-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-by: Changhyeok Bae <changhyeok.bae@gmail.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>