thermal: Add COMPILE_TEST support for i.MX8MM

Add COMPILE_TEST support to i.MX8MM thermal driver for better compile
testing coverage.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1583509057-8197-1-git-send-email-Anson.Huang@nxp.com