ARM: dts: Add STM32F746 MCU and STM32746g-EVAL board

The STMicrolectornics's STM32F746 MCU has the following main features:
 - Cortex-M7 core running up to @216MHz
 - 1MB internal flash, 320KBytes internal RAM (+4KB of backup SRAM)
 - FMC controller to connect SDRAM, NOR and NAND memories
 - Dual mode QSPI
 - SD/MMC/SDIO support
 - Ethernet controller
 - USB OTFG FS & HS controllers
 - I2C, SPI, CAN busses support
 - Several 16 & 32 bits general purpose timers
 - Serial Audio interface
 - LCD controller
 - HDMI-CEC
 - SPDIFRX

Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>