msp430 usci: TX operations via DMA

USCI UART A0 and A1 support DMA transmit via Msp430UartDmaA0C and
Msp430UartDmaA1C, respectively.  Since both components are implemented
using DMA channel 0, they cannot be both utilized in the same program.

A future parameterized DMA interface could be used to allow
serialization of DMA operations between UARTs using a single DMA
channel.