ARM: dts: am437x-sk: add explicit pinmux for both USB instances

This patch just makes USB[01]_DRVVBUS signal
explicitly muxed. Note that board already has
a discrete pulldown, so we're not adding any
pulls here.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>