arm64: tegra: Make the RTC a wakeup source on Jetson TX2

The RTC found on the MAX77620 PMIC can be used as a wakeup source on
Jetson TX2, which is useful to wake the system from suspend at a given
time.

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