arm64: tegra: Add RTC support on Tegra186

The RTC on Tegra186 is very similar to the RTC on earlier generations.
One notable exception is that the source clock is now the 32 kHz clock
instead of a dedicated RTC clock and the RTC alarm is a wake event and
can be used to wake the system from sleep.

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