MIPS: ralink: Add support for periodic timer irq

Adds a driver for the periodic timer found on Ralink SoC.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5682/


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