sunxi: Enable watchdog timer support by default

A watchdog helps recover from hangs or failure to boot an OS. It can
also be used by the sysreset framework to intentionally reset the
system. Now that a driver is available, let's enable this functionality
on sunxi boards.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>