drm: bridge: vga-dac: Add adi,adv7123 compatible string

drm: bridge: vga-dac: Add adi,adv7123 compatible string

The ADV7123 is a transparent VGA DAC. Unlike dumb VGA DACs it can be
controlled through a power save pin, and requires a power supply.
However, on most boards where the device is used neither the power save
signal nor the power supply are controllable.

To avoid developing a separate device-specific driver add an
"adi,adv7123" compatible entry to the dumb-vga-dac driver. This will
allow supporting most ADV7123-based boards easily, while allowing future
development of an adv7123 driver when needed without breaking backward
compatibility.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170302104728.7150-4-laurent.pinchart+renesas@ideasonboard.com