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>;
	};