ARM: dts: imx6ul-ccimx6ulsom: Fix the "coin" regulator name

As documented in pfuze100.yaml, the correct name for the regulator is
"coin", so change it to fix the following DT check warning:

imx6ul-ccimx6ulsbcexpress.dtb: pmic@8: regulators: 'vcoin' does not match any of the regexes:
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6ul-ccimx6ulsom.dtsi

@@ -158,7 +158,7 @@ ldo4_ext: vldo4 {
 				regulator-max-microvolt = <3300000>;
 			};
 
-			vcoin_chg: vcoin {
+			vcoin_chg: coin {
 				regulator-min-microvolt = <2500000>;
 				regulator-max-microvolt = <3300000>;
 			};