Allow the a51 descriptor file to be replaced with a C structure

[mithro]

Using Linux kernel headers for the "Chapter 9" definitions.

Allowing using C code to define USB descriptors rather than the horrible a51 ASM files.

See the following repos for this work;

• https://github.com/mithro/fx2lib-linux-headers
• https://github.com/mithro/fx2lib/tree/linux-descriptors
• https://github.com/mithro/HDMI2USB-misoc-firmware/blob/fx2-refactor-new/firmware/fx2/descriptors.c