spi: spi-mux: Add module info needed for autoloading

With the spi device table udev can autoload the spi-mux module in
the presence of an spi-mux device.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210721095321.2165453-1-u.kleine-koenig@pengutronix.de


Signed-off-by: Mark Brown <broonie@kernel.org>