xen/rpi4: implement watchdog-based reset

xen/rpi4: implement watchdog-based reset

The preferred method to reboot RPi4 is PSCI. If it is not available,
touching the watchdog is required to be able to reboot the board.

The implementation is based on
drivers/watchdog/bcm2835_wdt.c:__bcm2835_restart in Linux v5.9-rc7.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Tested-by: Roman Shaposhnik <roman@zededa.com>
CC: roman@zededa.com
(cherry picked from commit 25849c8b16f2a5b7fcd0a823e80a5f1b590291f9)