ARM: dts: imx6ul: use nvmem-cells for cpu speed grading

On i.MX6UL, accessing OCOTP directly is wrong because the ocotp clock
needs to be enabled first, so use the nvmem-cells binding instead.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6ul.dtsi

@@ -89,6 +89,8 @@ cpu0: cpu@0 {
 				      "pll1_sys";
 			arm-supply = <&reg_arm>;
 			soc-supply = <&reg_soc>;
+			nvmem-cells = <&cpu_speed_grade>;
+			nvmem-cell-names = "speed_grade";
 		};
 	};
 
@@ -942,6 +944,10 @@ tempmon_calib: calib@38 {
 				tempmon_temp_grade: temp-grade@20 {
 					reg = <0x20 4>;
 				};
+
+				cpu_speed_grade: speed-grade@10 {
+					reg = <0x10 4>;
+				};
 			};
 
 			lcdif: lcdif@21c8000 {