clk: cygnus: remove Cygnus dummy clock binding

Remove old Cygnus dummy clock binding document, as it's replaced by
Documentation/devicetree/bindings/clock/brcm,iproc-clocks.txt
Signed-off-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>

Documentation/devicetree/bindings/clock/bcm-cygnus-clock.txt

-Broadcom Cygnus Clocks
-
-This binding uses the common clock binding:
-Documentation/devicetree/bindings/clock/clock-bindings.txt
-
-Currently various "fixed" clocks are declared for peripheral drivers that use
-the common clock framework to reference their core clocks. Proper support of
-these clocks will be added later
-
-Device tree example:
-
-	clocks {
-		#address-cells = <1>;
-		#size-cells = <1>;
-		ranges;
-
-		osc: oscillator {
-			compatible = "fixed-clock";
-			#clock-cells = <1>;
-			clock-frequency = <25000000>;
-		};
-
-		apb_clk: apb_clk {
-			compatible = "fixed-clock";
-			#clock-cells = <0>;
-			clock-frequency = <1000000000>;
-		};
-
-		periph_clk: periph_clk {
-			compatible = "fixed-clock";
-			#clock-cells = <0>;
-			clock-frequency = <500000000>;
-		};
-	};