MIPS: ralink: add memory definition for RT305x

Populate struct soc_info with the data that describes our RAM window.

As memory detection fails on RT5350 we read the amount of available memory
from the system controller.

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5180/