dt-bindings: thermal: qoriq: Add optional clocks property

Some platforms like i.MX8M series SoCs have clock control for TMU,
add optional clocks property to the binding doc.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>