display: stm32: ltdc: fix RGB565/BGR565 format to match sample expectations

The LTDC RGB565 format corresponds to the PIXEL_FORMAT_BGR565 format.
Update the LTDC driver to advertise PIXEL_FORMAT_BGR565 as the supported
format instead of PIXEL_FORMAT_RGB565. This change ensures correct color
rendering in the sample display application and enables usage of the
video capture sample application.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>