USB: gadget: qti: Add sysfs node to unbind/bind gadget

In order to allow userspace change USB composition add
a sysfs node. It can be used by userspace to unbind
qti_usb gadget and enable new composition using configfs
if required.

Change-Id: I51b76fe935eabcbb132b77e447abb2c36f83ad83
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>