staging: greybus: mark PM functions as __maybe_unused
Enabling the arche platform for compile testing showed a harmless
warning with CONFIG_PM=n:
drivers/staging/greybus/arche-platform.c:632:12: error: 'arche_platform_resume' defined but not used [-Werror=unused-function]
drivers/staging/greybus/arche-platform.c:618:12: error: 'arche_platform_suspend' defined but not used [-Werror=unused-function]
This marks the functions as __maybe_unused to shut up the warnings.