watchdog: bcm2835_wdt: consider system-power-controller property

watchdog: bcm2835_wdt: consider system-power-controller property

Until now all Raspberry Pi boards used the power off function of the SoC.
But the Raspberry Pi 400 uses gpio-poweroff for the whole board which
possibly cannot register the poweroff handler because the it's
already registered by this watchdog driver. So consider the
system-power-controller property for registering, which is already
defined in soc/bcm/brcm,bcm2835-pm.txt .

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/1622981777-5023-3-git-send-email-stefan.wahren@i2se.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>