stm32f4: usb_midi: Introduce example MIDI firmware

Tested on STM32F4Discovery with Linux host using both amidi (raw
messages to/from device) and a synthesizer (note on/off).

This example depends upon changes to libopencm3 and should not be merged until the libopencm3 submodule includes the following:
https://github.com/libopencm3/libopencm3/pull/309
As the comment above the structure, it comes from one of the examples in the spec "Table B-2: MIDI Adapter Configuration Descriptor". I've shown it in hex because it is in hex in the spec.

I did sanity check this against other examples for the F4 to make sure the value is not lower than any other examples. As it happens every other example uses the same value (and quotes it in hex).

I plan to leave this as it is unles that makes you really unhappy.