board: Added board support package for Nucleo_F446ZE

Includes support for USB, CAN, ADC, DAC, and Arduino GPIO Map.
Future could support: ETM Trace (if Zephyr supports it), QSPI, FMC, DCMI

Initially based on closest BSP for the same SoC: nucleo_f446re
Extra peripherals added based on closest BSP for nucleo-144 board:
 nucleo_f429zi
Checked against nucleo-144 schematic
Documentation updated as best as I could
Arduino compatible pinmux for SPI, UART and I2C via Zio header

Tested:
  USB Device mode (samples/subsys/usb/console)
  Console via ST-link usart3 (samples/hello_world)
  user LEDs (samples/basic/blinky)
  scripts/twister --device-tests -p nucleo_f446ze
Not tested but should work: SPI, I2C, CAN, ADC, DAC
Not working yet: Quad-SPI (missing in dts/arm/st/f4/stm32f4.dtsi)

Signed-off-by: Tom Owen <tom.owen@zepler.net>