board: stm32: add stm32f469-discovery board support

board: stm32: add stm32f469-discovery board support

This board offers :

 _ STM32F469NIH6 microcontroller featuring 2 Mbytes of Flash memory
   and 324 Kbytes of RAM in BGA216 package
 _ On-board ST-LINK/V2-1 SWD debugger, supporting USB reenumeration capability:
     _ Mbed-enabled (mbed.org)
     _ USB functions: USB virtual COM port, mass storage, debug port
 _ 4 inches 800x480 pixel TFT color LCD with MIPI DSI interface and capacitive
   touch screen
 _ SAI Audio DAC, with a stereo headphone output jack
 _ 3 MEMS microphones
 _ MicroSD card connector
 _ I2C extension connector
 _ 4Mx32bit SDRAM
 _ 128-Mbit Quad-SPI NOR Flash
 _ Reset and wake-up buttons
 _ 4 color user LEDs
 _ USB OTG FS with Micro-AB connector
 _ Three power supply options:
 _ Expansion connectors and Arduino™ UNO V3 connectors

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>