mips: limit RAM size to 256MB on malta and qemu boards

This avoid crash when a bigger RAM size is requested (the devices are
mapped at 0x01000000).

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6419 c046a42c-6fe2-441c-8c8c-71466251a162
