thermal: sti: Ignore suspend/resume functions when !PM_SLEEP

Prevents build warning:

st_thermal.c:278:12:
    warning: ‘st_thermal_suspend’ defined but not used [-Wunused-function]
st_thermal.c:286:12:
    warning: ‘st_thermal_resume’ defined but not used [-Wunused-function]

Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>