usb: ci_hdrc_imx doc: fsl,usbphy is required

fsl,usbphy is no optional property. This patch moves it to the list of
required properties.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>