ARM: tegra: define DT bindings for and instantiate RTC

The Tegra RTC maintains seconds and milliseconds counters, and five alarm
registers. The alarms and other interrupts may wake the system from
low-power state.

Define a DT binding for this HW module, and add the module into the Tegra
device tree files.

Signed-off-by: Stephen Warren <swarren@nvidia.com>