How to design new driver for my own board

The major difference re GRBL & Linuxcnc is that Linuxcnc (what runs on the PC) is interprets the gcode and works out the timing & responds to realtime events.

With GRBL the PC sends gcode to the GRBL controller and that interprets the gcode.

There is no option (and so far it does that like it will ever be an option) to send gcode to an external controller.

I am going to install Linuxcnc on Orange Pi PC Plus. OPC send to FPGA controller
Speed and Axis coordinate in steps. FPGA controller has buffer for all parameters. According my calculations USB speed is enough to work in real time.

It's not speed that is the issue, it's regularity. Once you introduce a buffer you lose the ability for realtime response.
There's a reason that no USB development has been done for Linuxcnc.
Search the forum for questions related to USB and you'll get the same response over and over.
Mesa has one or two boards that have USB interfaces but none are used for Linuxcnc.

As previously stated, Ethernet, SPI or EPP are the way to go. USB maybe ok for control panels for non realtime input, but disconnection can be an issue.

I really suggest you look into it more, especially the difference between speed and regularity. You may also want to look to see if USB has any realtime drivers.

if you plan to use Orange PI why not just dedicate few pins and use SPI. RIO already has the driver and i believe works on other spartan 6 boards like a charm. you can use usb for firmware updates and non real time data (like pendant for example). usb has great speed but absolutely no guarantee time wise especially if your board will identify itself as vendor specific class. you can try to play with usb audio devices class, which seem to have better latency but you may still be disappointed after all that hard work you plan to do.

you can see here that even best fine tuned usb devices on RT kernel has a round trip close to 10ms
interfacinglinux.com/linux-compatible-audio-interfaces/
compared to stable 0.1-0.5ms with Ethernet or SPI