ahci_platform: Provide for vendor specific init

Some AHCI implementations may use Vendor Specific HBA[A0h, FFh]
and/or Port[70h, 7Fh] registers to 'prepare' for initialization.
For that, the platform needs memory mapped address of AHCI registers.

This patch adds the 'mmio' argument and reorders the call to
platform init function.

Signed-off-by: Jassi Brar <jassi.brar@samsung.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>