Documentation: dtb: xgene: Add hwmon dts binding documentation

This patch adds the APM X-Gene hwmon device tree node documentation.
Signed-off-by: NHoan Tran <hotran@apm.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/apm-xgene-hwmon.txt

+APM X-Gene hwmon driver
+
+APM X-Gene SOC sensors are accessed over the "SLIMpro" mailbox.
+
+Required properties :
+ - compatible : should be "apm,xgene-slimpro-hwmon"
+ - mboxes : use the label reference for the mailbox as the first parameter.
+	    The second parameter is the channel number.
+
+Example :
+	hwmonslimpro {
+		compatible = "apm,xgene-slimpro-hwmon";
+		mboxes = <&mailbox 7>;
+	};