drivers: Initial import of SPI low-level driver IF

This PR is a alternative for #1309 and closer to the existing driver interfaces.

The proposed interface should act as ground for discussion - I am not yet sure yet if this interface will be sufficient...

As a little bit of a novelty it introduces further methods to power-on and power-down SPI devices. This functionality should maybe also be included in any existing low-level driver, since our goal for RIOT is always to be as energy efficient as possible!
still works. SPI will always transmit a byte, even when you just want to receive one. Putting a `NULL` as parameter would just trigger the standard behavior of just sending a `0`, so even when you want to send a `0` it will work.