i2c/powermac: Register i2c devices from device-tree

This causes i2c-powermac to register i2c devices exposed in the
device-tree, enabling new-style probing of devices.

Note that we prefix the IDs with "MAC," in order to prevent the
generic drivers from matching. This is done on purpose as we only
want drivers specifically tested/designed to operate on powermacs
to match.

This removes the special case we had for the AMS driver, and updates
the driver's match table instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>