SPI additional implementations and fixes for CC2538

Contiki style apearance fixes to existing codes.

Added defines and Macros for quick switch between SPI0/1 (Pins etc.)

Enabled SPI clock for sleep and deep sleep modes

Added Slave functionality to SPI driver (Multi MCU connectivity for exchange of data through Master/Slave spi). 

Added interrupts and call backs to SPI driver

Added an example (examples/cc2538/spi) in which SPI (master/slave) can be tested.