Minor joybus subsystem cleanup

[meeq]

This is some groundwork for my upcoming Joypad subsystem that doesn't depend on any of the larger changes.

• Renames joybusinternal.h to joybus_internal.h for consistency with backtrace_internal.h, exception_internal.h, and rompak_internal.h
• Removes #include "libdragon.h" from joybus.c and replaces it with the individual header files that the module depends on.
• Wraps the JOYBUS_STATE_* macros in a Doxygen member group (it just looks nicer in the generated docs).
• Associates the Joybus Subsystem-related files with the joybus group in Doxygen
• Fixes missing #include <stdint.h> in interrupt.h