MIPS: ath79: add clock setup code for the QCA955X SoCs

The patch adds code to get various clock frequencies
from the PLLs used in the QCA955x SoCs.

Cc: Rodriguez, Luis <rodrigue@qca.qualcomm.com>
Cc: Giori, Kathy <kgiori@qca.qualcomm.com>
Cc: QCA Linux Team <qca-linux-team@qca.qualcomm.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4945/


Signed-off-by: John Crispin <blogic@openwrt.org>