aw_wdog: support Allwinner D1 watchdog

This device is present on the Allwinner D1-based SoCs. Without this
driver, the watchdog timeout will trigger a reset a few seconds after
control is given to the kernel.

Reviewed By: manu, mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34749