cpufreq: apple-soc: Switch to the lowest frequency on suspend

Without this, the CPUs are left in a random pstate. Since we don't
support deep idle yet (which powers down the CPUs), this results in
significantly increased idle power consumption in suspend.

Fixes: 6286bbb4 ("cpufreq: apple-soc: Add new driver to control Apple SoC CPU P-states")
Signed-off-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>