overlays: Make mcp342x run-time compatible

The order of processing of run-time overlays differs from that done by
the firmware. This means that certain parameter processing techniques
are not compatible with run-time use.  The mcp342x overlay is one such
overlay, but it is easy to change the implementation without changing
the interface.

See: https://www.raspberrypi.org/forums/viewtopic.php?f=107&t=258294



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