On Tegra, calling clk_set_rate on the CPU clock may call into the
regulator API.  If the regulator driver that controls the CPU
voltage rail has been suspended, this can lead to attempted
communication with a hardware block that has already been turned
off.

Adds a SUSPEND_PREPARE notification hook to drop the frequency to
the lowest possible during suspend.

Also adds 216MHz (off of PLLP) as the lowest CPU frequency, which
allows PLLX to be turned off.
Signed-off-by: NColin Cross <ccross@android.com>