Add DDR3 calibration code for i.MX6Q, i.MX6D and i.MX6DL. This code
fine-tunes the behavior of the MMDC controller in order to improve
the signal integrity and memory stability.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>