arm64: dts: ti: k3-am65-mcu: Add RTI watchdog entry

Add the DT entry for a watchdog based on RTI1. It requires additional
firmware on the MCU R5F cores to handle the expiry, e.g.
https://github.com/siemens/k3-rti-wdt

. As this firmware will also lock
the power domain to protect it against premature shutdown, mark it
shared.

Aligns us to the kernel's DT in this regard.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>