boards: 96b_carbon: adapt to the board configuration guidelines

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

Following changes are done:

- Enable exposed I2C2 in dts
- Only enable basic peripherals in board defconfig
- Add exposed peripherals in board Kconfig
- Update documentation for SPI

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