power: introduce SMB138X charger driver

This driver supports the SMB138X charger device.

Register access is provided by the parent device via regmap. Interrupts
are controlled by the parent device, and handlers are registered by the
SMB138X charger driver.

The power supply framework is used to communicate battery and usb
properties to userspace and other driver consumers such as fuel gauge
and USB.

VBUS regulator is registered for supporting OTG.

CRs-Fixed: 1023141
Change-Id: I36796332af667874c1246ec35984122d45de6938
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>