board: Add stm32h7 SoC, discovery and evaluation boards support

This patch adds support for stm32h7 soc family, stm32h743
discovery and evaluation boards.

For more information about STM32H7 series, please visit:
http://www.st.com/en/microcontrollers/stm32h7-series.html



Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>