ARM: dts: dra7-evm: Add extcon to USB2 port

Both ports on the dra7-evm and related boards can be used
as dual-role ports. Although we don't enable dual-role mode
for USB2 port let's add the necessary extcon bits to it.

Move the common portion of extcon_usb2 into dra7-evm-common.dtsi

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