MFC r273257:

MFC r273257:
 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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@273624 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f