dmaengine: ioatdma: Add intr_coalesce sysfs entry

We observed performance increase with DMA copy from memory
to MMIO by changing the interrupt coalescing value to 0.
The previous set value was projected on the C5xxx Xeon
platform and no longer holds true. Removing hard coded
value and providing a tune-able in sysfs in order to allow
user to tune this on a per channel basis. By default this
value will be set to 0.
Example of sysfs variable importing for interrupt coalescing
value from command line:
echo 5> /sys/devices/pci0000:00/0000:00:04.0/dma/dma0chan0/
quickdata/intr_coalesce

Reported-by: Nithin Sujir <nsujir@tintri.com>
Signed-off-by: Ujjal Singh <ujjal.singh@intel.com>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>