This patch provides the code to calibrate the DDR's
DQS to DQ signals (RDLVL).

It is based on:
VFxxx Controller Reference Manual, Rev. 0, 10/2016, page 1600
10.1.6.16.4.1 "Software Read Leveling in MC Evaluation Mode"

and NXP's community thread:
"Vybrid: About DDR leveling feature on DDRMC."
https://community.nxp.com/thread/395323Signed-off-by: NLukasz Majewski <lukma@denx.de>