dt-bindings: clock: Add documentation for X1830 bindings.

Add documentation for the clock bindings of the X1830 Soc from Ingenic.

Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: 周琰杰 (Zhou Yanjie) <zhouyanjie@wanyeetech.com>
Signed-off-by: Rob Herring <robh@kernel.org>