MIPS: Lantiq: Add support for setting PMU register on AR10 and GRX390

This adds support for setting the PMU register on the AR10 and GRX390.

Signed-off-by: Hauke Mehrtens <hauke.mehrtens@lantiq.com>
Acked-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11382/


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