libahci: Implement the capability to override the generic ahci interrupt handler.

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>