boards: arm: stm32h750b_dk: add support for display

ltdc devicetree definition & external SDRAM selection
for the display buffer

Signed-off-by: Philippe Cornu <philippe.cornu@foss.st.com>
Signed-off-by: Toon Stegen <toon@toostsolutions.be>
Signed-off-by: Abderrahmane Jarmouni <abderrahmane.jarmouni-ext@st.com>