watchdog: bcm2835_wdt: Fix module autoload

The commit 5e6acc3e ("bcm2835-pm: Move bcm2835-watchdog's DT probe
to an MFD.") broke module autoloading on Raspberry Pi. So add a
module alias this fix this.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>