Input: snvs_pwrkey - use __maybe_unused to hide pm functions

Input: snvs_pwrkey - use __maybe_unused to hide pm functions

The SNVS power key driver has suspend/resume functions that
are accessed using SIMPLE_DEV_PM_OPS, which hide the reference
when CONFIG_PM_SLEEP is not set, resulting in a warning about
unused functions:

drivers/input/keyboard/snvs_pwrkey.c:183:12: error: 'imx_snvs_pwrkey_suspend' defined but not used [-Werror=unused-function]
drivers/input/keyboard/snvs_pwrkey.c:194:12: error: 'imx_snvs_pwrkey_resume' defined but not used [-Werror=unused-function]

This adds __maybe_unused annotations to let the compiler know
it can silently drop the function definition.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>