drm/vc4: Add specific compatible strings for Cygnus.

Cygnus has V3D 2.6 instead of 2.1, and doesn't use the VC4 display
modules.  The V3D can be uniquely identified by the IDENT[01]
registers, and there's nothing to key off of for the display change
other than the lack of DT nodes for the display components, but it's
convention to have new compatible strings anyway.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Rob Herring <robh@kernel.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170428224223.21904-3-eric@anholt.net

drivers/gpu/drm/vc4/vc4_drv.c

@@ -335,6 +335,7 @@ static int vc4_platform_drm_remove(struct platform_device *pdev)
 
 static const struct of_device_id vc4_of_match[] = {
 	{ .compatible = "brcm,bcm2835-vc4", },
+	{ .compatible = "brcm,cygnus-vc4", },
 	{},
 };
 MODULE_DEVICE_TABLE(of, vc4_of_match);