dt-bindings: gk20a: Fix typo in compatible name

The correct compatible name is "nvidia,gk20a".
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>

Documentation/devicetree/bindings/gpu/nvidia,gk20a.txt

 NVIDIA GK20A Graphics Processing Unit
 
 Required properties:
-- compatible: "nvidia,<chip>-<gpu>"
+- compatible: "nvidia,<gpu>"
   Currently recognized values:
-  - nvidia,tegra124-gk20a
+  - nvidia,gk20a
 - reg: Physical base address and length of the controller's registers.
   Must contain two entries:
   - first entry for bar0