dt-bindings: clock: gxbb-clkc: Add GXL compatible variant

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/1490178747-14837-6-git-send-email-narmstrong@baylibre.com

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

@@ -5,7 +5,8 @@ controllers within the SoC.
 
 Required Properties:
 
-- compatible: should be "amlogic,gxbb-clkc"
+- compatible: should be "amlogic,gxbb-clkc" for GXBB SoC,
+	      or "amlogic,gxl-clkc" for GXL and GXM SoC.
 - reg: physical base address of the clock controller and length of memory
        mapped region.