pata_imx: expose module alias for loading from device-tree

Enable auto loading by udev when pata_imx is compiled as a module.

Signed-off-by: Niels de Vos <ndevos@redhat.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Tejun Heo <tj@kernel.org>