ARM: tegra: set up /aliases entries for RTCs

This ensures that the PMIC RTC provides the system time, rather than
the on-SoC RTC, which is not battery-backed.

tegra124-venice2.dts isn't touched yet since we haven't added any off-
SoC RTC device to its device tree.

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