MIPS: ralink: add memory definition to struct ralink_soc_info

Depending on the actual SoC we have a different base address as well as minimum
and maximum size for RAM. Add these fields to the per SoC structure.

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5179/