sata_fsl: add support for interrupt coalsecing feature

Adds support for interrupt coalescing feature to reduce interrupt events.
Provides a mechanism of adjusting coalescing count and timeout tick by sysfs
at runtime, so that tradeoff of latency and CPU load can be made depending
on different applications.

Signed-off-by: Qiang Liu <qiang.liu@freescale.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>