watchdog: Add support for K3 RTI watchdog

watchdog: Add support for K3 RTI watchdog

This is based on the Linux kernel driver for the RTI watchdog.

To actually reset the system on an AM65x, it requires firmware running
on the R5 that accepts the NMI and issues the actual system reset via
TISCI. Kind of an iTCO, except that this watchdog hardware has support
for no-way-out, and only for that.

On the J721E, reset works without extra firmware help when routing the
RTI interrupt via the ESM.

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