ARM: mx6: ddr: Add write leveling correction code

ARM: mx6: ddr: Add write leveling correction code

When the DDR calibration is enabled, a situation may happen that it
will fail on a few select boards out of a whole production lot. In
particular, after the first write leveling stage, the MPWLDECTRLx
registers will contain a value 0x1nn , for nn usually being 0x7f or
slightly lower.

What this means is that the HW write leveling detected that the DQS
rising edge on one or more bundles arrives slightly _after_ CLK and
therefore when the DDR DRAM samples CLK on the DQS rising edge, the
CLK signal is already high (cfr. AN4467 rev2 Figure 7 on page 18).

The HW write leveling then ends up adding almost an entire cycle (thus
the 0x17f) to the DQS delay, which indeed aligns it, but also triggers
subsequent calibration failure in DQS gating due to this massive offset.

There are two observations here:
- If the MPWLDECTRLx value is corrected from 0x17f to 0x0 , then the
  DQS gating passes, the entire calibration passes as well and the
  DRAM is perfectly stable even under massive load.
- When using the NXP DRAM calibrator for iMX6/7, the value 0x17f or so
  in MPWLDECTRx register is not there, but it is replaced by 0x0 as one
  would expect.

Someone from NXP finally explains why, quoting [1]:

    "
    Having said all that, the DDR Stress Test does something that we
    do not advertise to the users. The Stress Test iself looks at the
    values of the MPWLDECTRL0/1 fields before reporting results, and
    if it sees any filed with a value greater than 200/256 delay
    (reported as half-cycle = 0x1 and ABS_OFFSET > 0x48), the DDR
    Stress test will reset the Write Leveling delay for this lane
    to 0x000 and not report it in the log.

    The reason that the DDR Stress test does this is because a delay
    of more than 78% a clock cycle means that the DQS edge is arriving
    within the JEDEC tolerence of 25% of the clock edge. In most cases,
    DQS is arriving < 5% tCK of the SDCLK edge in the early case, and
    it does not make sense to delay the DQS strobe almost a full clock
    cycle and add extra latency to each Write burst just to make the
    two edges align exactly. In this case, we are guilty of making a
    decision for the customer without telling them we are doing it so
    that we don't have to provide the above explanation to every customer.
    They don't need to know it.
    "

This patch adds the correction described above, that is if the MPWLDECTRx
value is over 0x148, the value is corrected back to 0x0.

[1] https://community.nxp.com/thread/456246

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Eric Nelson <eric@nelint.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>