ata: ahci-imx: Fix MODULE_ALIAS

'ahci:' is an invalid prefix, preventing the module from autoloading.
Fix this by using the 'platform:' prefix and DRV_NAME.

Fixes: 9e54eae2

 ("ahci_imx: add ahci sata support on imx platforms")
Cc: stable@vger.kernel.org
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>