mx6: ddr: Restore ralat/walat in write level calibration

mx6: ddr: Restore ralat/walat in write level calibration

The current DDR write level calibration routine always overwrite
the ralat/walat fields to their maximum value, just save
the existing values at the beginning of the calibration routine
and restore it at the end.

In case the delay is estimated by the user to be more than one cycle the
walat should be configured according to that, this is not
automatically done.  From the i.MX6 RM:

   The user should read the results of the associated delay-line at
   MPWLDECTRL#[WL_DL_ABS_OFFSET#] and in case the user estimates that the
   reasonable delay may be above 1 cycle then the user should indicate it at
   MPWLDECTRL#[WL_CYC_DEL#]. Moreover the user should indicate it in
   MDMISC[WALAT] field. For example, if the result of the write leveling calibration
   is 100/256 parts of a cycle, but the user estimates that the delay is above 2 cycles
   then MPWLDECTRL#[WL_CYC_DEL#] should be configured to 2, so the total
   delay will be 2 and 100/256 parts of a cycle

Probably it would just possible to not overwrite the mdmisc register in
the first place, since this is not present in the write_level_calib() example
in NXP AN4467 nor in the i.MX6 RM (44.11.6.1 Hardware Write Leveling
Calibration).

Fixes: d339f16911c7 ("arm: imx6: Add DDR3 calibration code for MX6 Q/D/DL")
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>