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>

Documentation/devicetree/bindings/display/imx/fsl,imx-fb.txt

@@ -13,6 +13,8 @@ Required nodes:
 	Additional, the display node has to define properties:
 	- bits-per-pixel: Bits per pixel
 	- fsl,pcr: LCDC PCR value
+	A display node may optionally define
+	- fsl,aus-mode: boolean to enable AUS mode (only for imx21)
 
 Optional properties:
 - lcd-supply: Regulator for LCD supply voltage.