phy-sun4i-usb: Add support for peripheral-only mode

Use the new of_usb_get_dr_mode_by_phy() function to get the dr_mode
from the musb controller node instead of assuming that having an id_det
gpio means otg mode, and not having one means host mode.

Implement peripheral-only mode by adding a sun4i_usb_phy0_get_id_det
helper which looks at the dr_mode, always registering our extcon and
always monitoring vbus.

If dr_mode is not specified in the dts, do not register phy0 as we then
do not know how to treat it. This is actually a good thing as this means
we will not be registering phy0 on devices where the otg controller is
not enabled in the devicetree.

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