This patch implements the capability to override the generic AHCI
interrupt handler so that specific ahci drivers can implement their
own custom interrupt handler routines.  It also exports
ahci_handle_port_intr so that custom irq_handler implementations can
use it.

tj: s/ahci_irq_handler/irq_handler/ and updated description.
Signed-off-by: Suman Tripathi <stripathi@apm.com>
Signed-off-by: Tejun Heo <tj@kernel.org>