dt: Correct cam_reg GPIO assignments for CM1&3.

The standard instructions for wiring up cameras on CM1&3
assign GPIOs for both the shutdown and LED pins on the
camera connectors.
The assignment in DT matched the LED wiring, not the shutdown
line, so didn't control the camera power correctly.

Update the base DT accordingly.

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