Documentation: dtb: xgene: Add hwmon dts binding documentation

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>;

	};