dt-bindings: display: sitronix,st7571: add optional inverted property

dt-bindings: display: sitronix,st7571: add optional inverted property

Depending on which display that is connected to the controller, an "1"
means either a black or a white pixel.

The supported formats (R1/R2/XRGB8888) expects the pixels
to map against (4bit):
00 => Black
01 => Dark Gray
10 => Light Gray
11 => White

If this is not what the display map against, the controller has support
to invert these values.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250721-st7571-format-v2-2-159f4134098c@gmail.com
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>