The CPU power timer set up function was related to PMC register. Now moving
it to PMC driver. And it also help to clean up the PM related code later.

The timer was calculated based on the input clock of PMC. In this patch, we
also get the clock from DT.
Signed-off-by: Joseph Lo <josephl@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>