thermal: spear: use __maybe_unused for PM functions

The spear thermal driver hides its suspend/resume function conditionally
based on CONFIG_PM, but references them based on CONFIG_PM_SLEEP, so
we get a warning if the former is set but the latter is not:

thermal/spear_thermal.c:58:12: warning: 'spear_thermal_suspend' defined but not used [-Wunused-function]
thermal/spear_thermal.c:75:12: warning: 'spear_thermal_resume' defined but not used [-Wunused-function]

This removes the #ifdef and instead uses a __maybe_uninitialized
annotation to avoid the warning and improve compile-time coverage.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>