dtoverlays: Add option for cam0 to camera-mux-Nport overlays

Seeing as the mux can be connected to either CAM/DISP1 or
CAM/DISP0 on a Pi5, add a cam0 override to allow configuration
of which is used. Default (as with all camera overlays) is CAM/DISP1.

The overlay does NOT update the camera regulator used by all the
sensors as doing so would be a nightmare. The Arducam mulitplexer
boards these overlays are initially supporting seem to tie the
regulator GPIO for all the sensors high anyway.
If it was viewed as necessary, then creating an additional
regulator that listed cam[01]_reg as the parent should work.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>