ARM: dts: exynos: Add support for UHS-I SD cards on Arndale Octa

The Exynos5420's Mobile Storage Host supports SD cards in UHS-I standard
(SD specification v3.0), with 1.8 V signaling in SD UHS DDR50.  Adjust
the regulator and add necessary capability properties.  Change the SDR
and DDR timings to match values in Insignal v3.4 Android kernel.

Tested with SD UHS-I card in SD UHS DDR50 mode.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>