smb137b: Add smb137b charger driver

The smb137b chip from SUMMIT is a Switch Mode charger chip capable
of charging through USB source. It can also provide VBUS when the
system is a host.  The driver exposes the charger properties via
the power_supply class. It also interacts with the usb driver for
insertion/removal notifications and charge current information.
The driver also turns off charging and switches to providing VBUS
when told to do so by USB driver.

Change-Id: I0ace31b9e1c7780bc9b40d5a2572340dc97966ac
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>