ARM: dts: AM437x: switch to compatible pinctrl

Now that ti,am437-padconf is available, switch over to that compatible
property. Retain pinctrl-single for legacy support.

While at it, mark the pinctrl as interrupt controller so that it can
be used with interrupts-extended property for wakeup events.

Signed-off-by: Nishanth Menon <nm@ti.com>