the SPI_ASYNCH feature has been already activated for STM32F4.
This patchset makes it supported on all STM32 families by:
- moving spi_s structure at family level instead of board level
- using the F4 spi_api.c reference implementation and making it a common
stm_spi_api.c file which makes maintenance a lot easier.
- the only part that needs to be implemented for each family is the computation
of the clock frequency input to the spi peripheral which is not the same
accross families. So this is what remains in the spi_api.c of each family.

Because of the introduction of the common file, all the above modifications
needs to be done at once.