MIPS: sead3: allow cmdline/env to change memory size using memsize param

if the user sets memsize parameter in commandline or bootloader
environment, we use it to modify the built-in dtb memory size

Signed-off-by: Qais Yousef <qais.yousef@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6207/