dt-bindings: thermal: imx8mm-thermal: Add binding doc for i.MX8MM

Add thermal binding doc for Freescale's i.MX8MM Thermal Monitoring Unit.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1582947862-11073-1-git-send-email-Anson.Huang@nxp.com

Documentation/devicetree/bindings/thermal/imx8mm-thermal.txt

+* Thermal Monitoring Unit (TMU) on Freescale i.MX8MM SoC
+
+Required properties:
+- compatible : Must be "fsl,imx8mm-tmu".
+- reg : Address range of TMU registers.
+- clocks : TMU's clock source.
+- #thermal-sensor-cells : Should be 0. See ./thermal.txt for a description.
+
+Example:
+tmu: tmu@30260000 {
+	compatible = "fsl,imx8mm-tmu";
+	reg = <0x30260000 0x10000>;
+	clocks = <&clk IMX8MM_CLK_TMU_ROOT>;
+	#thermal-sensor-cells = <0>;
+};