phy-sun4i-usb: Add support for phy_set_mode

Together with some musb sunxi glue changes this allows run-time dr_mode
switching support via the "mode" musb sysfs attribute.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>