power: smb1351: Update the driver to use extcon

The USB driver on 4.9 kernel uses the extcon framework
for communication, update the driver to support this.

This also requires defining the USB power-supply class
in the charger driver.

Change-Id: I3d3fc78368b84362776b119653666abe12abc018
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>