cpufreq: apple-soc: Allow per-SoC configuration of APPLE_DVFS_CMD_PS1

Support for SoC that has a different APPLE_DVFS_CMD_PS1 will be added soon,
so modify the driver first to allow it to be configured per-SoC.

Signed-off-by: Nick Chan <towinchenmi@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>