display: stm32-ltdc: clarify pixel-format property of lcd_controller

So far the LTDC driver has not be using the pixel-format property
defined in the lcd_controller bindings. This property describes the
format of the panel connected to the LTDC output. It is not to be
confused with the format of the LTDC layers (aka format of data stored
into the framebuffer). The LTDC accepts various input formats while in
most cases only RGB888 is supported for output. RGB666 is also supported
by enabling Dithering however it is not supported currently by the
driver.
Enforce verification of the pixel-format given to the LTDC device-tree
by checking that it matches with the platform supported formats.

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