memory: Add DMC driver for Exynos5422

Add driver for Exynos5422 Dynamic Memory Controller.  The driver
provides support for dynamic frequency and voltage scaling for DMC and
DRAM.  It supports changing timings of DRAM running with different
frequency.  There is also an algorithm to calculate timings based on
memory description provided in DT.

Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>