MFC r273247:
 Fix the watchdog/watchdog man pages.

 The default timeout is 128 seconds.
MFC r273248:
 AM335x FDT: add an entry for the watchdog.
MFC r273256:
 AM335X FDT: use the omap3-wdt string for compatibility with Linux.
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.
MFC r273258:
 Make the ti_mbox and ti_pruss drivers optional.
MFC r273261:
 Remove an unused mutex.