boards: arm: nucleo_f756zg: Added SPI support

Added support for SPI 1 available from the arduino connector (plus an
extra gpio). Tested against samples/sensor/bme280.

Added warnings in documentation, pinmux and dts highlighting a potential
conflict if using SPI_1 and on-board ethernet at the same time.

Signed-off-by: AJ Palmer <ajpcode@hotmail.com>