STM32 DMAv2 driver.

Driver capability:

- The driver supports the STM32 enhanced DMA controller found on F2, F4 and
  F7 sub-families.
- Support for automatic the channel selection.
- Support for cache flushing and invalidation.

The file registry must export:

STM32_ADVANCED_DMA          - TRUE not used by the DMA drivers but other
                              drivers use it to enable checks on DMA
                              channels. Probably will be removed in the
                              future.
STM32_HAS_DMAx              - Support for DMA unit "x" (1..2).
STM32_DMAx_CHn_HANDLER      - Vector name for channel "n" (0..7).
STM32_DMAn_CHx_NUMBER       - Vector number for channel "n" (0..7).
STM32_DMA_CACHE_HANDLING    - TRUE if the device requires explicit cache
                              handling on DMA buffers.