Updates to CC2538 SPI

In my first iteration I had a specific SPI chip select line,
which initially made sense as there is a CS pin that the SSI will
toggle for you. However, if you want multiple SPI devices that
solution does not scale. Instead, I removed the hard coded CS line and
added a function which allows drivers to configure spi to use their CS
line before the call SPI_READ or SPI_WRITE.