soc/tegra: pmc: Fix verification of valid partitions

The Tegra power partitions are referenced by numerical IDs which are the
same values programmed into the PMC registers for controlling the
partition. For a given device, the valid partition IDs may not be
contiguous and so simply checking that an ID is not greater than the
maximum ID supported may not mean it is valid. Fix this by checking if
the powergate is defined in the list of powergates for the Tegra SoC.

Add a helper function for checking valid powergates and use where we
need to verify if the powergate ID is valid or not.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>