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: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1584674791-9717-1-git-send-email-Anson.Huang@nxp.com