DMC: Exynos5: Enable update mode for DREX controller

As per Exynos5800 UM ver 0.00 section 17.13.2.1
CONCONTROL register bit 3 [update_mode], Exynos5800 does not
support the PHY initiated update. And it is recommanded to
set this field to 1'b1 during initialization. This patch sets this bit.
Applying MC-initiated mode makes DDL tracking ON, that helps in
compensate MIF voltage variation.

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