Basic DAC Driver, implementation on stm32f4discovery

- Creation of DAC Driver, much like the current ADC Driver.
- Implementation of the DAC Driver for the stm32f4discovery board.

dac_init(dac_t dev, dac_precision_t precision)
Initializing a DAC Device with a given precision, configures the Device and Channels. 

dac_write(dac_t dev, uint8_t channel, uint16_t value)
Writes a value on a specific channel for a device.

dac_poweron(dac_t dev)
Enables a given DAC Device.

dac_poweroff(dac_t dev)
Disables a given DAC Device.

uint16_t dac_map(dac_t dev, int value, int min, int max)
Maps a integer value between interval min and max, to a valid integer to write onto the dac with respect to the selected precision.

uint16_t dac_mapf(dac_t dev, float value, float min, float max)
Maps a float value between interval min and max, to a valid integer to write onto the dac with respect to the selected precision.
We need to careful with these pins, so far they are configured for ADC (PA4) and SPI SCK (PA6). Have a look at [this table](https://docs.google.com/spreadsheets/d/11iqdux3K1-7FPZChufNgpChwENkgCiLrnKJPoCcJxdg/edit#gid=1428302540) and check if you can find other pins. If not we need to see if we can re-arrange the pin mapping for the f4discovery board