Use only 3 bits for a GPIO port pin number

This is enough for 8 bits.
Replace uses of "USART" with "UART"

LM3S3749 does not have USARTs.
Add SSI support using libi2c

Only SPI mode with one fixed speed setting is supported for now.
SSI ports are not initialized on board bootup, so this should be
done by an application. SPI interface can use any GPIO pin as
slave select, the pin number should be supplied as I2C address.
The pin used as SS should be initialized by the application or
driver before use, SSI module does not attempt initialization
itself.