soc/tegra: pmc: Ensure powergate is available when powering on

The function tegra_power_sequence_power_up() is a public function used
to power on a partition. When this function is called, we do not check
to see if the partition being powered up is valid/available. Fix this
by checking to see that the partition is valid/available.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>