usb: phy: Introduce one extcon device into usb phy

Usually usb phy need register one extcon device to get the connection
notifications. It will remove some duplicate code if the extcon device
is registered using common code instead of each phy driver having its
own related extcon APIs. So we add one pointer of extcon device into
usb phy structure, and some other helper functions to register extcon.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>