ARM: dts: am57xx-idk: Support VBUS detection on USB2 port

VBUS detection is available on the board via the palmas PMIC.
Use extcon-palmas driver instead of extcon-gpio-usb driver.

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