drivers: Initial import of SPI low-level driver IF
drivers: fixes to the SPI low-level interface
driver: updated SPI driver interface
[SQUASH ME] added fixed speed values
[SQUASH ME] removed UNDEFINED from device list
[SQUASH ME] fixed signature of spi_transfer_reg()
[SQUASH ME] added begin_transaction, fixed doc
Merge pull request #1311 from haukepetersen/add_driver_spi_if

drivers: Initial import of SPI low-level driver IF
drivers: Provide basic interface for all network devices
drivers: Make general IEEE 802.15.4 driver part of net_dev