thermal: imx: fix for dependency on cpu-freq

The thermal driver is a standalone driver for monitoring SoC temperature
by enabling thermal sensor, so it can be enabled even when CONFIG_CPU_FREQ
is NOT set. So remove the dependency with CPU_THERMAL.

Introduce dummy function of legacy cooling register/unregister to make
thermal driver probe successfully when CONFIG_CPU_FREQ is NOT set.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>