EADC, dummy: Add a driver based on the platform compatible for these systems

On a system with multiple edac controllers, something needs to co-ordinate
the information from each source, to give edac a single view of the system.

Add a driver that spots these systems during subsys_initcall_sync() and
configures both devices. This requires the standalone driver to be
structured so that its setup routines can be used as a library by
another driver. Not obvious from this example is the combined driver
has to be able to provide a callback to be used when reporting an
error to edac.

This lets us leave the DT and drivers unmodified.

We also need to stop the standalone drivers from probing. Instead of
trying to force something here and now, or destroying the platform device,
we use the new driver_override helper.

This gives us:
| EDAC MC0: Giving out device to module skeleton_edac controller
| skeleton COMBINED ctl name: DEV skeleton COMBINED dev name (INTERRUPT)

Signed-off-by: James Morse <james.morse@arm.com>