dts: bindings: usb: Add maximum-speed property
dts: bindings: usb: Add maximum-speed property

Add maximum-speed property to usb node.
It configures USB controllers to work up to a specific speed.
Valid arguments are "super-speed", "high-speed", "full-speed"
and "low-speed".

Signed-off-by: Yannis Damigos <bda985a2727ac24c41652a294292988d6fc7dc73@gmail.com>

galak/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,punitvara/zephyr,galak/zephyr,finikorg/zephyr,explora26/zephyr,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,punitvara/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,punitvara/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,explora26/zephyr,ldts/zephyr,explora26/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,ldts/zephyr,nashif/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,galak/zephyr