ARM: dts: Declare Pi400 and CM4 have no audio pins

The audio_pins node is left as a placeholder for the audremap overlay,
and it must have (empty) brcm,function and brcm,pins properties
otherwise it will be rejected by the pinctrl driver.

See: https://www.raspberrypi.org/forums/viewtopic.php?f=98&t=301891



Signed-off-by: Phil Elwell <phil@raspberrypi.com>