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, but sometimes this can be convenient, example:
´´´c
#define SOME_COMMAND 0xab
spi_transfer_byte(SPI_0, SOME_COMMAND, NULL);
```
vs.
```c
#define SOME_COMMAND 0xab
char x = SOME_COMMAND;
spi_transfer_bytes(SPI_0, &x, NULL, 1); // working
spi_transfer_bytes(SPI_0, SOME_COMMAND, NULL, 1); // not working...
```