dt: bindings: Add bindings for Cirrus Logic CS89x0 ethernet chip

Add device tree binding documentation details for Cirrus Logic
CS8900/CS8920 ethernet chip.
Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/cirrus,cs89x0.txt

+* Cirrus Logic CS8900/CS8920 Network Controller
+
+Required properties:
+- compatible	: Should be "cirrus,cs8900" or "cirrus,cs8920".
+- reg		: Address and length of the IO space.
+- interrupts	: Should contain the controller interrupt line.
+
+Examples:
+	eth0: eth@10000000 {
+		compatible = "cirrus,cs8900";
+		reg = <0x10000000 0x400>;
+		interrupts = <10>;
+	};