ARM: dts: stm32: add stm32f769I & stm32f746 discovery board support

Stm32f769I & stm32f746 are MCUs of stm32f7 family. Here are the major
specs of the two boards:

stm32f769I discovery board:
	- Cortex-M7 core @216MHz
	- 2MB mcu internal flash
	- 512KB internal sram
	- 16MB sdram memory
	- 64MB qspi flash memory
	- 4 inch wvga LCD-TFT Display

stm32f746 discovery board:
	- Cortex-M7 core @216MHz
	- 1MB mcu internal flash
	- 320KB internal sram
	- 8MB sdram memory
	- 16MB qspi flash memory
	- 4.3 inch 480x272 LCD-TFT display

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>