video: fbdev: imxfb: support AUS mode

Some displays require setting AUS mode in the LDCD AUS Mode Control
Register to work with the imxfb driver. Like the value of the Panel
Configuration Register, the AUS mode setting depends on the display
mode.

Allow setting AUS mode from the device tree by adding a boolean
property. Make this property optional to keep the DT ABI stable.
AUS mode can be set only on imx21 and compatible chipsets.

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