OMAP4 has two watchdog timers timer 2/MPU and timer 3/IVA.

OMAP4 has two watchdog timers timer 2/MPU and timer 3/IVA.
Previously the device tree only described one but now it describes both.
Do not attempt to set the global variable that points to a softc or
register a watchdog if this has already been done.

Fixes rebooting on the OMAP4 based PandaBoard-ES with a device tree from
linux 4.15-rc1.