MIPS: ralink: Add support for systick timer found on newer ralink SoC

Newer Ralink SoC (MT7620x and RT5350) have a 50KHz clock that runs independent
of the SoC master clock. If we want to automatic frequency scaling to work we
need to use the systick timer as the clock source.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5669/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>