Documentation: dt: socfpga: add reset-cells property

To be able to use the reset-controller framework, the property
#reset-cells
is mandatory.

Signed-off-by: Steffen Trumtrar <[e-mail]>
Signed-off-by: Dinh Nguyen <[e-mail]>

Documentation/devicetree/bindings/arm/altera/socfpga-reset.txt

···
 Required properties:
 - compatible : "altr,rst-mgr"
 - reg : Should contain 1 register ranges(address and length)
+- #reset-cells: 1
 
 Example:
 	 rstmgr@ffd05000 {
+		#reset-cells = <1>;
 		compatible = "altr,rst-mgr";
 		reg = <0xffd05000 0x1000>;
 	};