boards: st: add pllsai for ltdc & mipi-dsi support on stm32f469 disco Extension of support of pllsai for display configuration of stm32f469 discovery board. Enable Display panel through LTDC & DSI-HOST blocks. Enable Touch screen. Enable FMC/SDRAM for Framebuffer. Signed-off-by: Philippe Peurichard <p.peurichard@gmail.com>