MIPS: ralink: adds clkdev code

These SoCs have a limited number of fixed rate clocks. Add support for the
clk and clkdev api.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4894/