arc: emsdp/iotdk: Use standard compatible string for USB no-op PHY

arc: emsdp/iotdk: Use standard compatible string for USB no-op PHY

The standard compatible string is "usb-nop-xceiv", use it.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexey Brodkin <alexey.brodkin@synopsys.com>
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Jean-Jacques Hiblot <jjhiblot@ti.com>
Cc: Murali Karicheri <m-karicheri2@ti.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Cc: uboot-imx <uboot-imx@nxp.com>