devicetree/bindings: Add binding for operator panel on FSP machines

Add a binding to Documentation/devicetree/bindings/powerpc/opal
(oppanel-opal.txt) for the operator panel which is present on IBM
Power Systems machines with FSPs.
Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Documentation/devicetree/bindings/powerpc/opal/oppanel-opal.txt

+IBM OPAL Operator Panel Binding
+-------------------------------
+
+Required properties:
+- compatible : Should be "ibm,opal-oppanel".
+- #lines     : Number of lines on the operator panel e.g. <0x2>.
+- #length    : Number of characters per line of the operator panel e.g. <0x10>.
+
+Example:
+	oppanel {
+		compatible = "ibm,opal-oppanel";
+		#lines = <0x2>;
+		#length = <0x10>;
+	};