MIPS: ralink: allow manual memory override

RT5350 relies on the bootloader setting up the memc correctly. On some boards
the setup is incorrect leading to 32 MB being available but only 16 MB being
recognized. Allow these boards to manually override the memory range.

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/8004/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>