thermal: imx_thermal: Use __maybe_unused instead of CONFIG_PM_SLEEP

Use __maybe_unused for power management related functions instead
of #if CONFIG_PM_SLEEP to simplify the code.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1583139266-23615-1-git-send-email-Anson.Huang@nxp.com