ARM: dts: am57xx-idk: Enable dual role for USB2 port

Dual-role support was added in v4.12. We should be using
it for USB2 port on the am57xx-idk.

Cc: <stable@vger.kernel.org>        [4.16+]
Reported-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>