ARM: dts: imx: Add missing #phy-cells to usb-nop-xceiv

"usb-nop-xceiv" is using the phy binding, but is missing #phy-cells
property. This is probably because the binding was the precursor to the phy
binding.

Fixes the following warning in i.MX dts files:

Warning (phys_property): Missing property '#phy-cells' in node ...

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx7d.dtsi

@@ -109,6 +109,7 @@
 		compatible = "usb-nop-xceiv";
 		clocks = <&clks IMX7D_USB_PHY2_CLK>;
 		clock-names = "main_clk";
+		#phy-cells = <0>;
 	};
 
 	fec2: ethernet@30bf0000 {