bindings: spmi: spmi-pmic-arb: Add description for "qcom,mid"

Add a description for the "qcom,mid" property which corresponds
to the SPMI master ID of the SPMI PMIC arbiter controller. It is
most useful in systems with multiple masters, e.g. when there are
multiple SoCs connected to a single SPMI bus.

Change-Id: I376e6f569f9bbea44ba3930480a330fb1dd2c2de

bindings/spmi/qcom,spmi-pmic-arb.txt

@@ -43,6 +43,8 @@ Optional properties:
     cell 3: the requested peripheral interrupt (0-7)
     cell 4: interrupt flags indicating level-sense information, as defined in
             dt-bindings/interrupt-controller/irq.h
+- qcom,mid : SPMI master ID of this controller. Supported values
+	     are 0, 1, 2 and 3.
 
 Consumer node optional properties:
 - qcom,pmic-arb : phandle for an spmi-pmic-arb device.  This can be used by
@@ -63,6 +65,7 @@ Example:
 
 		qcom,ee = <0>;
 		qcom,channel = <0>;
+		qcom,mid = <0>;
 
 		#address-cells = <2>;
 		#size-cells = <0>;