dt-bindings: thermal: imx8mm-thermal: Document optional nvmem-cells

The TMU TASR, TCALIVn, TRIM registers must be explicitly programmed with
calibration values from OCOTP. Document optional phandle to OCOTP nvmem
provider.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@kernel.org>