boards: arm: fix i2c pin and add SPI to docs

When I added this board recently I mistakenly placed
the Arduino SCL pin (Default Zephyr pin) as PB6
when it should be PB8.

The SPI node is now properly set to SPI2 peripheral
instead of SPI1. SPI2 is the most suitable to follow
the silkscreen on the printed circuit board.

Added arduino_gpio to yaml.

Signed-off-by: Guðni Már Gilbert <gudni.m.g@gmail.com>