dt-bindings: thermal: imx8mm-thermal: Add support for i.MX8MP

Add thermal binding doc for Freescale's i.MX8MP Thermal Monitoring Unit.
Signed-off-by: NAnson Huang <Anson.Huang@nxp.com>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1584674791-9717-1-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".
+- compatible : Must be "fsl,imx8mm-tmu" or "fsl,imx8mp-tmu".
 - reg : Address range of TMU registers.
 - clocks : TMU's clock source.
-- #thermal-sensor-cells : Should be 0. See ./thermal.txt for a description.
+- #thermal-sensor-cells : Should be 0 or 1. See ./thermal.txt for a description.
 
 Example:
 tmu: tmu@30260000 {