Only 48MHz, 72MHz, 96MHz and 120 MHz system clocks derived from an
external crystal are suitable for the usb peripheral, as the internal
oscillator is not stable enough. Also the usb-prescaler only supports
division by 1 (48MHZ), 1.5(72MHz), 2(96MHz) and 2.5(120Mhz).
120Mhz is also out of spec and not added here.
Instead of using the HAL functions we can just use the defines from the
board support for the longan nano that comes with the nuclei-sdk. Also
we move some includes and defines to the header file.