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>

Documentation/devicetree/bindings/thermal/qoriq-thermal.txt

@@ -23,6 +23,7 @@ Required properties:
 Optional property:
 - little-endian : If present, the TMU registers are little endian. If absent,
 	the default is big endian.
+- clocks : the clock for clocking the TMU silicon.
 
 Example: