arm64: dts: ti: k3-am64-mcu: Add pinctrl

Add the definition of the pinctrl for the MCU domain.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20210818111747.88569-1-christian.gmeiner@gmail.com