staging: greybus: arche-apb-ctrl: fix unused warnings on resume/suspend

When CONFIG_PM_SLEEP is disabled, SIMPLE_DEV_PM_OPS does not use
arche_apb_ctrl_resume and arche_apb_ctrl_suspend functions:

drivers/staging/greybus/arche-apb-ctrl.c:478:12: warning:
‘arche_apb_ctrl_resume’ defined but not used [-Wunused-function]
 static int arche_apb_ctrl_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~~~
drivers/staging/greybus/arche-apb-ctrl.c:464:12: warning:
‘arche_apb_ctrl_suspend’ defined but not used [-Wunused-function]
 static int arche_apb_ctrl_suspend(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~~~~

Adding __maybe_unused to the declaration of these functions removes the
warnings.

Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Acked-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>