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