ARM: shmobile: Document DT bindings for Product Register

Add device tree binding documentation for the Product Register (PRR),
which provides product and revision information on most Renesas ARM
SoCs.

Signed-off-by: Geert Uytterhoeven <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Simon Horman <[e-mail]>

Documentation/devicetree/bindings/arm/shmobile.txt

···
     compatible = "renesas,silk", "renesas,r8a7794"
   - Wheat
     compatible = "renesas,wheat", "renesas,r8a7792"
+
+
+Most Renesas ARM SoCs have a Product Register that allows to retrieve SoC
+product and revision information.  If present, a device node for this register
+should be added.
+
+Required properties:
+  - compatible: Must be "renesas,prr".
+  - reg: Base address and length of the register block.
+
+
+Examples
+--------
+
+	prr: chipid@ff000044 {
+		compatible = "renesas,prr";
+		reg = <0 0xff000044 0 4>;
+	};