dt-bindings: ap806: add the cluster clock node in the syscon file

Document the device tree binding for the cluster clock controllers found
in the Armada 7K/8K SoCs.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Link: https://lkml.kernel.org/r/20190710134346.30239-2-gregory.clement@bootlin.com
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>