soc/tegra: pmc: Export tegra_powergate_power_on()

tegra_powergate_sequence_power_up() powers up partition and also enables
clock & reset. However, if a controller like PCIe have multiple clocks
& resets and they need to be enabled in a sequence, driver must use
standalone function tegra_powergate_power_on() to power up partition.

Export tegra_powergate_power_on() to allow Tegra controller drivers to
unpower gate partition independent to clock & reset.

Signed-off-by: Manikanta Maddireddy <mmaddireddy@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Thierry Reding <treding@nvidia.com>