MLK-13508-1 mx6sllevk: Update DDR script to v2.2

DDR script is updated to v2.2 to fix potential DQS gating issue. That DQS sampling
may have problem after enabling the SDE_0/SDE_1 in MDCTL.

Changes:
    -Based on V2.1.1, move the "Read DQS Gating Disable" to the step after
     "MR setting", to avoid potential DDR initializaiton failures
     (especially in Plugin Mode).

File:
    http://compass.freescale.net/livelink/livelink?func=ll&objid=235701297&objAction=browse&sort=name&viewType=1



Test:
    Passed stress test on two boards.

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 8adb838b)