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