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.

REFERENCE:
https://github.com/nxp-imx/linux-imx/commit/979556f1521a835a059de3b117b9c6c6642c7d58