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!
True, how about a parameter, that tells the callback which byte in a transaction is transferred, e.g. byte 0, 1 ... n, which is reset each time the cs line was toggled!?