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!
In the current definition, the callback would need to keep state and "parse" the received bytes for command, address and data. As you mention this is fine for simple transactions like register based access where you can expect a two to three byte stream, but when receiving a larger number of bytes, callback would need to know about chip select signal I think. To determine if transaction is finished.