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

Documentation/devicetree/bindings/display/brcm,bcm-vc4.txt

@@ -5,7 +5,7 @@ with HDMI output and the HVS (Hardware Video Scaler) for compositing
 display planes.
 
 Required properties for VC4:
-- compatible:	Should be "brcm,bcm2835-vc4"
+- compatible:	Should be "brcm,bcm2835-vc4" or "brcm,cygnus-vc4"
 
 Required properties for Pixel Valve:
 - compatible:	Should be one of "brcm,bcm2835-pixelvalve0",
@@ -54,7 +54,7 @@ Required properties for VEC:
 		  See bindings/interrupt-controller/brcm,bcm2835-armctrl-ic.txt
 
 Required properties for V3D:
-- compatible:	Should be "brcm,bcm2835-v3d"
+- compatible:	Should be "brcm,bcm2835-v3d" or "brcm,cygnus-v3d"
 - reg:		Physical base address and length of the V3D's registers
 - interrupts:	The interrupt number
 		  See bindings/interrupt-controller/brcm,bcm2835-armctrl-ic.txt

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);

drivers/gpu/drm/vc4/vc4_v3d.c

@@ -450,6 +450,7 @@ static int vc4_v3d_dev_remove(struct platform_device *pdev)
 
 static const struct of_device_id vc4_v3d_dt_match[] = {
 	{ .compatible = "brcm,bcm2835-v3d" },
+	{ .compatible = "brcm,cygnus-v3d" },
 	{ .compatible = "brcm,vc4-v3d" },
 	{}
 };