ARM: dts: exynos: Fix LDO13 min values on Odroid XU3/XU4/HC1

From Odroid XU3/XU4/HC1 schematics the LDO13 regulator for SD2, can be
set on 1.8V or 2.8V so the minimal value should be fixed to 1.8V.  This
is necessary to support UHS-I tuning (otherwise card won't be detected
during boot).

Signed-off-by: Anand Moon <linux.amoon@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>