libahci: Allow drivers to override start_engine

Allwinner A10 and A20 ARM SoCs have an AHCI sata controller which needs a
special register to be poked before starting the DMA engine.

This register gets reset on an ahci_stop_engine call, so there is no other
place then ahci_start_engine where this poking can be done.

This commit allows drivers to override ahci_start_engine behavior for use by
the Allwinner AHCI driver (and potentially other drivers in the future).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>