arm64: tegra: p2888: Enable on-die RTC

The on-die RTC isn't hooked up to a backup battery, so it isn't useful
to track time across reboots, but as long as power remains enabled, it
keeps track of time accurately and can be used to wake the system from
sleep, for example.

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