ARM: dts: imx: Add dummy PHYs for HSIC-only USB controllers

Some SOCs in the i.MX6 family have a USB host controller that is
only capable of the HSIC interface and has no on-board PHY.

To be able to use these controllers, we need to add "usb-nop-xceiv"
dummy PHYs.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>