stm32f7: add support for stm32f769 disco board

This board support stm32f7 family device stm32f769-I with 2MB internal Flash &
512KB RAM.
STM32F769 lines offer the performance of the Cortex-M7 core (with double
precision floating point unit) running up to 216 MHz.

To compile for stm32f769 board, use same defconfig as stm32f746-disco,
the only difference is to pass "DEVICE_TREE=stm32f769-disco".

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
cc: Christophe KERELLO <christophe.kerello@st.com>