power: smb135x-charger: support external battery thermistor bias voltage

smb135x charger chips can accept an external supply for thermistor
biasing. Add code to enable that external regulator if present.

Change-Id: Ie6691cf3eca6c2dc85f438819f7436b44c9c2aca
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>