arm64: tegra: Make the RTC a wakeup source on Jetson Nano and TX1

The RTC found on the MAX77620 PMIC can be used as a wakeup source on
Jetson Nano and TX1, which is useful to wake the system from suspend
at a given time.

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