ath79: D-Link DAP-2xxx (QCA953x): convert ath9k caldata to nvmem

Convert the calibration data reference for the ath9k radio to an
nvmem-cell, replacing the downstream mtd-cal-data property.

Since the 'art' label is no longer used, it can be dropped.

Cc: Sebastian Schaper <openwrt@sebastianschaper.net>
Signed-off-by: Sander Vanheule <sander@svanheule.net>

target/linux/ath79/dts/qca953x_dlink_dap-2xxx.dtsi

@@ -42,10 +42,18 @@
 				read-only;
 			};
 
-			art: partition@ff0000 {
+			partition@ff0000 {
 				label = "art";
 				reg = <0xff0000 0x10000>;
 				read-only;
+
+				compatible = "nvmem-cells";
+				#address-cells = <1>;
+				#size-cells = <1>;
+
+				cal_ath9k: calibration@1000 {
+					reg = <0x1000 0x440>;
+				};
 			};
 		};
 	};
@@ -54,5 +62,6 @@
 &wmac {
 	status = "okay";
 
-	mtd-cal-data = <&art 0x1000>;
+	nvmem-cells = <&cal_ath9k>;
+	nvmem-cell-names = "calibration";
 };