Enabled SPI for the TM4C129x

[tcaxle]

PR for #56

Code tested on the EK-TM4C1294XL with SSI0 on GPIO port A in SDIO mode.

Further testing recommended but basic functionality is confirmed.

Closes: #56

[tcaxle]

See picoscope output for the Hello World example here:

[thejpster]

Please run cargo fmt. One query but otherwise looks great, and fantastic to see the scope screenshot - very reassuring!