smb138x-charger: enable the watchdog timer when parallel is enabled

If software becomes unresponsive then the battery could be overcharged by
the parallel charger. Enable the watchdog so that when the bite timer
expires then charging will be disabled by hardware.

Change-Id: I82febbc28c05563d052c6eed034adc817df39790
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>