Made Tiva C SPI frequency configurable
Made Tiva C SPI frequency configurable

The tiva_c::spi::Spi struct now accepts an SpiConf config object.
Currently this is limited to configuring which peripheral to use (SSI0 –
SSI3) and what frequency to run the bus at.

Frequency is calculated by set_frequency() which was shamelessly
borrowed from the lpc17xx ssp.rs module.

ntwyman/zinc,mcoffin/zinc,ntwyman/zinc,ntwyman/zinc,mcoffin/zinc,richo/zinc,hackndev/zinc,hackndev/zinc,hackndev/zinc,richo/zinc,richo/zinc,mcoffin/zinc,mcoffin/zinc,hackndev/zinc,richo/zinc,mcoffin/zinc,ntwyman/zinc,richo/zinc,hackndev/zinc,ntwyman/zinc