boards: arm: nucleo_f756zg: Created new board and soc definition

Added board definitions for nucleo_f756zg. Features include gpio,
pinmux, uart (ST Zio, ST-Link and Arduino Uno v3 interfaces).

Added basic documentation and some soc definitions for the
stm32 f756XX soc.

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