ata: Remove the AHCI_HFLAG_EDGE_IRQ support from libahci.

The flexibility to override the irq handles in the LLD's are already
present, so controllers implementing a edge trigger latch can
implement their own interrupt handler inside the driver.  This patch
removes the AHCI_HFLAG_EDGE_IRQ support from libahci and moves edge
irq handling to ahci_xgene.

tj: Minor update to description.

Signed-off-by: Suman Tripathi <stripathi@apm.com>
Signed-off-by: Tejun Heo <tj@kenrel.org>