Exynos5420: DMC: Add software read leveling

Exynos5420: DMC: Add software read leveling

Sometimes Read DQ and DQS are not in phase. Since, this
phase shift differs from board to board, we need to
calibrate it at DRAM init phase, that's read DQ calibration.
This patch adds SW Read DQ calibration routine to compensate
this skew.

Signed-off-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Akshay Saraswat <akshay.s@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>