dt-bindings: clock: add compatible variant for the Meson-AXG

Update the documentation to support clock driver for the Amlogic's
Meson-AXG SoC.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>

Documentation/devicetree/bindings/clock/amlogic,gxbb-clkc.txt

@@ -5,8 +5,11 @@ controllers within the SoC.
 
 Required Properties:
 
-- compatible: should be "amlogic,gxbb-clkc" for GXBB SoC,
-	      or "amlogic,gxl-clkc" for GXL and GXM SoC.
+- compatible: should be:
+		"amlogic,gxbb-clkc" for GXBB SoC,
+		"amlogic,gxl-clkc" for GXL and GXM SoC,
+		"amlogic,axg-clkc" for AXG SoC.
+
 - reg: physical base address of the clock controller and length of memory
        mapped region.