ata: ceva: Make RxWaterMark value as module parameter

This patch updates the driver to make Rx Fifo water mark value
as a module parameter.

Signed-off-by: Anurag Kumar Vulisha <anuragku@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Tejun Heo <tj@kernel.org>