ARM: dts: am43xx: Introduce MUX_MODE9 for pinctrl

Some pins on AM43XX support MODE9 for the pinctrl settings so add a
binding to describe this.

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