boards: 96b_nitrogen: adapt to the board configuration guidelines

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

Following changes are done:

- Enable I2C0 and SPI0 in board dts
- Add exposed peripherals in board Kconfig
- Update board yaml
- Remove config option to select board
- Update documentation for UART, I2C and SPI

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