ata: pata_pxa: dmaengine conversion

The PXA architecture was offered a slave dmaengine support. As a
consequence the direct DMA registers are progressively replaced by
dmaengine support.

This makes the pata_pxa change, which brings this driver to almost a
generic ATA 40-wires driver relying on dmaengine for transfers.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Tejun Heo <tj@kernel.org>