dm: panel: Add a panel uclass

LCD panels can usefully be modelled as their own uclass. They can be probed
(which powers them up ready for use). If they have a backlight, this can be
enabled.

Signed

dm: panel: Add a panel uclass

LCD panels can usefully be modelled as their own uclass. They can be probed
(which powers them up ready for use). If they have a backlight, this can be
enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>

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