ARM: tegra: colibri_t20: iris: add missing aliases

Add rtc0 being the ultra low-power I2C one as found on the carrier board
and the 3rd UART being NVIDIA's UARTB.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>