watchdog: rti_wdt: Add support for loading firmware

watchdog: rti_wdt: Add support for loading firmware

To avoid the need of extra boot scripting on AM65x for loading a
watchdog firmware, add the required rproc init and loading logic for the
first R5F core to the watchdog start handler. In case the R5F cluster is
in lock-step mode, also initialize the second core. The firmware itself
is embedded into U-Boot binary to ease access to it and ensure it is
properly hashed in case of secure boot.

One possible firmware source is https://github.com/siemens/k3-rti-wdt.

The board is responsible for providing the firmware as additional
loadable via the U-Boot fit image. The driver will pick up its location
from /fit-images/k3-rti-wdt-firmware then.

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