ARM: dts: am57xx: sbc-am57x: add usb vbus pinmux

usb1_drvvbus pin is used to Drive-VBUS enable to external charge
pump/power switch.

Add a pinmux for that pin.

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>