boards: 96b_carbon_nrf51: adapt to the board configuration guidelines

As per default board configuration guidelines, this commit updates
96Boards Carbon nRF51 board.

Following changes are done:

- Enable I2C0 in board dts
- Update board yaml
- Remove config option to select board
- Update documentation for SPI

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>