ARM: dts: am57xx-beagle-x15: Add extcon nodes for USB

On this EVM, 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).

NOTE: the ports on this board cannot switch roles. They are
configured either host or peripheral by hardwiring the
respective ID pins.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>