Added SPI driver for CC2538

This uses the core/dev/spi.h header and implements the spi_init()
function and the various macros for SPI operation. ssi.h contains all of
the register locations and information.

This implementation is not very versatile, mostly because I don't how to
make it flexible in the contiki system. It supports pin muxing for the
four spi pins, but other than that picks sensible defaults.

The SPI macros (like SPI_READ()) are defined in
platform/cc2538dk/contiki-conf.h. Is there a better place for them? They
are CPU specific so I think they should be in cpu/cc2538, but there
doesn't seem to be a logical place given that spi.h is already defined.