usb: ehci-mx6: Do not fail when 'reg' is not found

usb: ehci-mx6: Do not fail when 'reg' is not found

Unlike imx6, on imx7 the USB PHY is described as:

	usbphynop1: usbphynop1 {
		compatible = "usb-nop-xceiv";
		clocks = <&clks IMX7D_USB_PHY1_CLK>;
		clock-names = "main_clk";
		#phy-cells = <0>;
	};

which does not have the 'reg' property.

Do not return an error when the 'reg' property is not found
for the USB PHY.

This fixes USB gadget regression on a imx7s-warp board.

Successfully tested the "ums 0 mmc 0" command on two boards:
imx7s-warp and imx6dl-pico-pi.

Signed-off-by: Fabio Estevam <festevam@denx.de>