- Add missing `volatile` to DMA descriptor, as memory is also accessed by the
DMA without knowledge of the compiler
- Dropped `__attribute__((packed))` from DMA descriptor
- The DMA descriptor fields need to be aligned on word boundries to
properly function
- The compiler can now more efficiently access the fields (safes ~300 B ROM)
- Moved the DMA descriptor struct and the flags to `periph_cpu.h`
- This allows Doxygen documentation being build for it
- Those types and fields are needed for a future PTP implementation
- Renamed DMA descriptor flags
- They now reflect to which field in the DMA descriptor they refer to, so
that confusion is avoided
- Added documentation to the DMA descriptor and the corresponding flags
a5dbec33d9