MIPS: Add new GIC clocksource.

Add new clocksource that uses the counter present on the MIPS
Global Interrupt Controller.

Signed-off-by: Steven J. Hill <sjhill@mips.com>
Patchwork: http://patchwork.linux-mips.org/patch/4681/


Signed-off-by: John Crispin <blogic@openwrt.org>