powerpc: Add documentation for CPM GPIO banks

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+Every GPIO controller node must have #gpio-cells property defined,
+this information will be used to translate gpio-specifiers.
+
+On CPM1 devices, all ports are using slightly different register layouts.
+Ports A, C and D are 16bit ports and Ports B and E are 32bit ports.
+
+On CPM2 devices, all ports are 32bit ports and use a common register layout.
+
+Required properties:
+- compatible : "fsl,cpm1-pario-bank-a", "fsl,cpm1-pario-bank-b", 
+  "fsl,cpm1-pario-bank-c", "fsl,cpm1-pario-bank-d",
+  "fsl,cpm1-pario-bank-e", "fsl,cpm2-pario-bank"
+- #gpio-cells : Should be two. The first cell is the pin number and the
+  second cell is used to specify optional paramters (currently unused).
+- gpio-controller : Marks the port as GPIO controller.
+
+Example of three SOC GPIO banks defined as gpio-controller nodes:
+
+	CPM1_PIO_A: gpio-controller@950 {
+		#gpio-cells = <2>;
+		compatible = "fsl,cpm1-pario-bank-a";
+		reg = <0x950 0x10>;
+		gpio-controller;
+	};
+
+	CPM1_PIO_B: gpio-controller@ab8 {
+		#gpio-cells = <2>;
+		compatible = "fsl,cpm1-pario-bank-b";
+		reg = <0xab8 0x10>;
+		gpio-controller;
+	};
+
+	CPM1_PIO_E: gpio-controller@ac8 {
+		#gpio-cells = <2>;
+		compatible = "fsl,cpm1-pario-bank-e";
+		reg = <0xac8 0x18>;
+		gpio-controller;
+	};