Add apple_gmux_config struct containing operations and data specific to
each mux type.

This is in preparation for adding a third, MMIO based, gmux type.
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Orlando Chamberlain <orlandoch.dev@gmail.com>
Link: https://lore.kernel.org/r/20230303112842.3094-3-orlandoch.dev@gmail.comSigned-off-by: Hans de Goede <hdegoede@redhat.com>