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>

arch/arm/boot/dts/exynos4412-odroid-common.dtsi

@@ -389,9 +389,9 @@ ldo20_reg: LDO20 {
 			};
 
 			ldo21_reg: LDO21 {
-				regulator-name = "LDO21_3.3V";
-				regulator-min-microvolt = <3300000>;
-				regulator-max-microvolt = <3300000>;
+				regulator-name = "TFLASH_2.8V";
+				regulator-min-microvolt = <2800000>;
+				regulator-max-microvolt = <2800000>;
 				regulator-boot-on;
 			};