usb: dwc3: core: add dt support for dwc3 core

Added dt support for dwc3 core and update the documentation with
device tree binding information. Getting a PHY is now done using
devm_usb_get_phy_by_phandle() for dt boot.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>