cpufreq: armada-8k: Avoid excessive stack usage

In some build configurations (e.g. x86 allmodconfig), 2 cpu_mask
variables exceeds the max stack frame size:

drivers/cpufreq/armada-8k-cpufreq.c:203:1: error: the frame size of 2128 bytes is larger than 2048 bytes

Fix it by making "cpus" static which is fine given that module init is
only called once.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>