Add a driver for the TI watchdog.

The TI watchdog timer is present on BeagleBone's.  Since 2014, U-Boot
has been booting the BeagleBone with the watchdog enabled.  We need to
disable it on boot to avoid a spurious reset.
The timer isn't exactly precise, but it will do as a watchdog.  This
is also a reflection of the watchdog(9) API.

In the future, we could handle interrupts, but the watchdog(9) API
needs to be a bit smarter before that can happen.

Differential Revision:	https://reviews.freebsd.org/D965
Reviewed by:	andrew
MFC after:	1 week
Relnotes:	yes
Make the ti_mbox and ti_pruss drivers optional.

MFC after:	1 week