Add support for GD32VF103 chips [rebase]

[katyo]

This is rebase of original PR #25.

I tested it with test class example.
Also I tested CDC ACM made using usbd_serial crate (usb-otg-workspace#7)

[katyo]

@Disasm I would like integrate OTG FS support directly to gd32vf103xx-hal crate.
Any chace to merge this PR or original one soon?

[Disasm]

The biggest problem so far is testing: I'd like the PR to be tested before merge, not only with GD32V, but also with other STM32 chips to avoid regressions. I have the hardware needed, but I need time which I don't have enough at the moment.

[katyo]

@Disasm I developing CI solution for running tests on hardware.
Could you list microcontrollers/eval boards needed for that project?

[Disasm]

I used the following chips/boards:
STM32F401 (WeAct Black Pill)
STM32F407 (XCore407I) - external PHY support testing
STM32F429 (NUCLEO-F429ZI)
STM32F446 (NUCLEO-F446ZE)
STM32F723 (32F723EDISCOVERY) - internal HS+FS PHY testing
GD32VF103 (Longan Nano)

Also I don't have a board for STM32H7xx, but it would be nice to test it as well (#4, stm32-rs/stm32h7xx-hal#108)

[katyo]

@Disasm Thanks!

Currently I have only the following:

• STM32F401 (WeAct Black Pill)
• STM32F407 (XCore407I) - external PHY support testing
• STM32F411 (Black Pill)
• STM32F429 (NUCLEO-F429ZI)
• STM32F446 (NUCLEO-F446ZE)
• STM32F723 (32F723EDISCOVERY) - internal HS+FS PHY testing
• GD32VF103 (Longan Nano)