video: stm32: stm32_ltdc: align framebuffer on 2MB

Align the framebuffer size on MMU_SECTION_SIZE in kernel, = max 2MB for
LPAE for armV7, to avoid issue with the simple frame buffer activation,
when U-Boot add a reserved memory in the kernel device tree to preserve
the splash screen until Linux driver initialization.

See Linux documentation for details:
Documentation/devicetree/bindings/display/simple-framebuffer.yaml

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>