spi: stm32: avoid ifdef CONFIG_PM for pm callbacks

Avoid CONFIG_PM preprocessor check for pm suspend/resume
callbacks and identify the functions with __maybe_unused.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Link: https://lore.kernel.org/r/1615545329-5496-1-git-send-email-alain.volmat@foss.st.com


Signed-off-by: Mark Brown <broonie@kernel.org>