MIPS: ralink: add RT3352 register defines

Add a few missing defines that are needed to make USB and clock detection work
on the RT3352.

Signed-off-by: John Crispin <blogic@openwrt.org>
Acked-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5166/