drm: atmel-hlcdc: Support inverting the pixel clock polarity

On the SoC host controller, the pixel clock can be:
* standard: data is launched on the rising edge
* inverted: data is launched on the falling edge

Some panels may need the inverted option to be used so let's support
this DRM flag.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230609144843.851327-1-miquel.raynal@bootlin.com