MIPS: ralink: add rt2880 wmac clock

Register the wireleass mac clock on rt2880. This is required by the wifi driver.

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/8006/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>