MIPS: ath79: Use local variables for clock rates

Use local variables for ref, cpu, ddr and ahb
rates in SoC specific clock init functions.

The patch has no functional changes, it is
an interim change in preparation of the next
patch.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5781/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>