MIPS: ralink: Probe clocksources from OF

Make plat_time_init() call clocksource_of_init() allowing the systick cevt
to load.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/5670/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>