apm821xx: wndr4700: add preliminary drivetemp sensor

This patch prepares the WNDR4700 to use the HDD sensor for
the thermal zone. While the kernel's thermal.txt device-tree
binding documentation files talks about supporting multiple
sensors for a zone. This sadly is NOT the case. Even the most
current upstream kernels (5.6-rc) supports just >one< sensor
per zone: (driver/base/of-thermal.c:886)
| * REVIST: for now, the thermal framework supports only
| * one sensor per thermal zone. Thus, we are considering
| * only the first two values as slope and offset.

I do hope that this warning will prevent others wasteing time
on trying to figure out why their multi-sensor thermal-zones
definitions are not working as specified.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

target/linux/apm821xx/dts/netgear-wndr4700.dts

@@ -36,13 +36,25 @@
 
 			thermal-sensors = <&temp0 1>;
 
+			/*
+			 * REVISIT:
+			 *
+			 * Add the <&drive_temp>; sensor there and wire up
+			 * the coefficients = <1 1>; property.
+			 *
+			 * Note: The kernel does not yet support more than
+			 * one sensor (see of_thermal.c's function:
+			 * thermal_of_build_thermal_zon()). Once this is
+			 * implemented.
+			 */
+
 			trips {
 				/*
 				 * Once the thermal governers are a bit smarter
 				 * and do hysteresis properly, we can disable
 				 * the fan when the HDD and CPU has < 39 C.
 				 */
-				cpu_alert0: cpu-alert0 {
+				cpu_alert0: board-alert0 {
 					temperature = <25000>;
 					hysteresis = <2000>;
 					type = "active";
@@ -145,6 +157,17 @@
 
 &SATA1 {
 	status = "okay";
+
+	/*
+	 * This drive may have a temperature sensor with a
+	 * thermal zone we can use for thermal control of the
+	 * chassis temperature using the fan.
+	 */
+
+	drive_temp: sata-port@0 {
+		reg = <0>;
+		#thermal-sensor-cells = <0>;
+	};
 };
 
 &USBOTG0 {

target/linux/apm821xx/image/Makefile

@@ -191,7 +191,7 @@ TARGET_DEVICES += netgear_wndap660
 define Device/netgear_wndr4700
   DEVICE_VENDOR := NETGEAR
   DEVICE_MODEL := Centria N900 WNDR4700/WNDR4720
-  DEVICE_PACKAGES := badblocks block-mount e2fsprogs \
+  DEVICE_PACKAGES := badblocks block-mount e2fsprogs kmod-hwmon-drivetemp \
 	kmod-dm kmod-fs-ext4 kmod-fs-vfat kmod-usb-ledtrig-usbport \
 	kmod-md-mod kmod-nls-cp437 kmod-nls-iso8859-1 kmod-nls-iso8859-15 \
 	kmod-nls-utf8 kmod-usb3 kmod-usb-dwc2 kmod-usb-storage \