OMAP2+: cpufreq: support clamping initial max frequency to board-provided value

This is useful to allow boards to define "overclocking" OPPs without
having them be automatically enabled when the cpufreq driver loads.

Change-Id: I29d94981d3827280d8b1bc4cd11dc60f8de14db7
OMAP3: encore: clamp initial max CPU clock frequency to nominal

This prevents the system from using out-of-spec overclocking OPPs until
userspace explicitly enables them.

Change-Id: I64b182aa2b51dc3a34e1d0da0cdea7c9b21ae11b