MIPS: ralink: adds support for MT7620 SoC family

Add support code for mt7620 SOC.

The code detects the SoC and registers the clk / pinmux settings.

Signed-off-by: John Crispin <blogic@openwrt.org>
Acked-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5177/