musb/allwinner: add support for configuring phy as well as device mode

At least on Orange Pi PC Plus even the host mode does not work without
enabling the phy and setting it to the host mode.

The driver will now parse dr_mode property and will try to configure
itself and the phy accordingly.
OTG mode is not supported yet, so it is treated as the device / peripheral
mode.

The phy is enabled -- powered on -- only for the host mode.

The device mode requires support from a phy driver, e.g., aw_usbphy on
Allwinner platform.
aw_usbphy does not support the device mode, so it cannnot work yet.

MFC after: 6 weeks