dt-bindings: display: imx: entry for AUS mode

Allow setting AUS mode for a display from the device tree. Use an
optional boolean property. AUS mode can be set only on imx21 and
compatible chipsets.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Acked-by: Rob Herring <robh@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>