MIPS: ralink: add RT5350 sdram register defines

Add a few missing defines that are needed to make memory detection work on the
RT5350.

Signed-off-by: John Crispin <blogic@openwrt.org>
Acked-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5169/