phy-sun4i-usb: Add extcon support for the otg phy (phy0)

The sunxi musb glue needs to know if a host or normal usb cable is plugged
in, add extcon support so that the musb glue can monitor the host status.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>