vxr7200: add sysfs node for proximity control

Adding sysfs node that could be used to turn off Display power supplies
when Proximity is off, and turns on Display when Proximity is on.
Also, added USB event handler to handle USB cable connect/disconnect
events.

Change-Id: Ib45def0282efc5a58e09b2b7c9a363d1a2d13ac5
Signed-off-by: Rajesh Bharathwaj <rajeshbharathwaj@codeaurora.org>