ahci: mtk: Change driver name to ahci-mtk

The driver name "ahci" is already used by the ahci platform driver.
This leads to the following error:
Error: Driver 'ahci' is already registered, aborting...

Change the name to ahci-mtk to fix this.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>