boards: arm: stm32: basic support for STM32F769I-DISCO board

This patch adds basic support and for the STM32F769I-DISCO board
with GPIO, USART support. Two USARTs are enabled.The GPIOs are
configured for the 4 LEDs and one button on the board.

Signed-off-by: Yong Jin <jinyong.iot@foxmail.com>