ARM: dts: dra7x-evm: beagle-x15: Fix USB Peripheral

Now that we have EXTCON_USB_GPIO queued for v4.1, revert
commit addfcde7 ("ARM: dts: dra7x-evm: beagle-x15: Fix USB Host")

On these EVMs, the USB cable state has to be determined via the
ID pin tied to a GPIO line. We use the gpio-usb-extcon driver
to read the ID pin and the extcon framework to forward
the USB cable state information to the USB driver so the
controller can be configured in the right mode (host/peripheral).

Gets USB peripheral mode to work on this EVM.

Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>