dt-bindings: move backlight bindings under leds

Backlights are generally a subtype of LEDs at least from a software
point of view if not always electrically. Move the bindings from the
video directory to underneath the leds dir.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>