dtoverlays: Add overlay cm-swap-i2c0 to swap buses on i2c0mux

The legacy camera stack wiring instructions connect the cameras
to the opposite sets of GPIOs compared to the arrangement
more normally used with libcamera on all other platforms.

Add an overlay to allow easy swapping of the assignments so
that the legacy wiring can be used with libcamera.

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