ARM: imx: add select on ARCH_MXC for cpufreq support

Move ARCH_HAS_CPUFREQ, ARCH_HAS_OPP and PM_OPP on ARCH_MXC so that
the user can enable the cpufreq support for iMX6Q and/or iMX6SL.

Signed-off-by: John Tobias <john.tobias.ph@gmail.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>