ahci: qoriq: Update the default Rx watermark value

The PTC[RXWM] sets the watermark value for Rx FIFO. The default
value 0x20 might be insufficient for some hard drives. If the
watermark value is too small, a single-cycle overflow may occur
and is reported as a CRC or internal error in the PxSERR register.
Updated the value to 0x29 according to the validation test.
All LS platforms are affected.

Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>
Signed-off-by: Tejun Heo <tj@kernel.org>