MIPS: ath79: Use a helper function to get system clock rates

MIPS: ath79: Use a helper function to get system clock rates

The ath79 platform uses similar code to get the
rate of various clocks during init. Separate the
similar code into a new helper function and use
that to avoid code duplication.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5778/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>