mfd: atmel-flexcom: Fix compilation warning

Fix "unused variable 'atmel_flexcom_pm_ops' [-Wunused-const-variable]"
compilation warning by using __maybe_unused on PM ops.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211207111515.814653-1-claudiu.beznea@microchip.com