MIPS: Lantiq: Add clock detection for grx390 and ar10

This add detection of some clocks 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/11385/


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