boards: 96b_carbon_nrf51: add support for 96Boards Carbon nRF51 chip

This adds support for the nRF51 chip on the board.

If you'd like to run Zephyr on the STM32F4 chip on Carbon, you need to
use the 96b_carbon board instead.

The current SPI Bluetooth protocol only uses 5 wires, so we use the
remaining pin as UART TX.

Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>
Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>