Initial Stm32f4 spi driver

as this is just the code for SPI port SPI_0, the pins will not get > 7 and it  is still ARF[0]. for the SPI_1 case, there are the pins PB12-PB15 preconfigured. for them i use ARF[1]. but to be save i can implement conditions for that, so that a user can maybe setup other pins for spi