ARM: dts: exynos: Lower SD card interface voltage to 2.8 V on Odroid X/X2/U3

Odroid X/X2/U3 schematics say that SD card vmmc regulator
(LDO21/TFLASH) operates on 2.8 V. Mainline U-Boot uses that value as
well and 2.8 V is common on Exynos-based boards. Additionally use some
descriptive name for this regulator.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>