arm64: dts: imx8mn-ddr4-evk: Align regulator names with schema

Device tree schema expects regulator names to be lowercase.  Changing to
lowercase has multiple effects:
1. LDO6 supply is now properly configured, because regulator driver
   looks for supplies by lowercase name,
2. User-visible names via sysfs or debugfs are now lowercase,
2. dtbs_check warnings are fixed:

    pmic@4b: regulators:LDO1:regulator-name:0: 'LDO1' does not match '^ldo[1-6]$'
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-By: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mn-ddr4-evk.dts

@@ -60,7 +60,7 @@ pmic@4b {
 
 		regulators {
 			buck1_reg: BUCK1 {
-				regulator-name = "BUCK1";
+				regulator-name = "buck1";
 				regulator-min-microvolt = <700000>;
 				regulator-max-microvolt = <1300000>;
 				regulator-boot-on;
@@ -69,7 +69,7 @@ buck1_reg: BUCK1 {
 			};
 
 			buck2_reg: BUCK2 {
-				regulator-name = "BUCK2";
+				regulator-name = "buck2";
 				regulator-min-microvolt = <700000>;
 				regulator-max-microvolt = <1300000>;
 				regulator-boot-on;
@@ -79,14 +79,14 @@ buck2_reg: BUCK2 {
 
 			buck3_reg: BUCK3 {
 				// BUCK5 in datasheet
-				regulator-name = "BUCK3";
+				regulator-name = "buck3";
 				regulator-min-microvolt = <700000>;
 				regulator-max-microvolt = <1350000>;
 			};
 
 			buck4_reg: BUCK4 {
 				// BUCK6 in datasheet
-				regulator-name = "BUCK4";
+				regulator-name = "buck4";
 				regulator-min-microvolt = <3000000>;
 				regulator-max-microvolt = <3300000>;
 				regulator-boot-on;
@@ -95,7 +95,7 @@ buck4_reg: BUCK4 {
 
 			buck5_reg: BUCK5 {
 				// BUCK7 in datasheet
-				regulator-name = "BUCK5";
+				regulator-name = "buck5";
 				regulator-min-microvolt = <1605000>;
 				regulator-max-microvolt = <1995000>;
 				regulator-boot-on;
@@ -104,7 +104,7 @@ buck5_reg: BUCK5 {
 
 			buck6_reg: BUCK6 {
 				// BUCK8 in datasheet
-				regulator-name = "BUCK6";
+				regulator-name = "buck6";
 				regulator-min-microvolt = <800000>;
 				regulator-max-microvolt = <1400000>;
 				regulator-boot-on;
@@ -112,7 +112,7 @@ buck6_reg: BUCK6 {
 			};
 
 			ldo1_reg: LDO1 {
-				regulator-name = "LDO1";
+				regulator-name = "ldo1";
 				regulator-min-microvolt = <1600000>;
 				regulator-max-microvolt = <3300000>;
 				regulator-boot-on;
@@ -120,7 +120,7 @@ ldo1_reg: LDO1 {
 			};
 
 			ldo2_reg: LDO2 {
-				regulator-name = "LDO2";
+				regulator-name = "ldo2";
 				regulator-min-microvolt = <800000>;
 				regulator-max-microvolt = <900000>;
 				regulator-boot-on;
@@ -128,7 +128,7 @@ ldo2_reg: LDO2 {
 			};
 
 			ldo3_reg: LDO3 {
-				regulator-name = "LDO3";
+				regulator-name = "ldo3";
 				regulator-min-microvolt = <1800000>;
 				regulator-max-microvolt = <3300000>;
 				regulator-boot-on;
@@ -136,7 +136,7 @@ ldo3_reg: LDO3 {
 			};
 
 			ldo4_reg: LDO4 {
-				regulator-name = "LDO4";
+				regulator-name = "ldo4";
 				regulator-min-microvolt = <900000>;
 				regulator-max-microvolt = <1800000>;
 				regulator-boot-on;
@@ -144,7 +144,7 @@ ldo4_reg: LDO4 {
 			};
 
 			ldo6_reg: LDO6 {
-				regulator-name = "LDO6";
+				regulator-name = "ldo6";
 				regulator-min-microvolt = <900000>;
 				regulator-max-microvolt = <1800000>;
 				regulator-boot-on;