[ARM] tegra: cpufreq: Disable cpufreq during suspend

[ARM] tegra: cpufreq: Disable cpufreq during suspend

Adds a SUSPEND_PREPARE notification hook to drop the frequency to
the lowest possible during suspend.  This prevents the cpufreq driver
from attempting regulator calls after suspend has started - the
regulator api can call into drivers that have already been suspended.
Also adds 216MHz (off of PLLP) as the lowest CPU frequency, which
allows PLLX to be turned off.

Change-Id: I46a7f88610ce35b6f761a557905861f79bc4df0b
Signed-off-by: Colin Cross <ccross@google.com>