Input: migor-ts - mark PM functions as __maybe_unused

The migor touchscreen driver is now available for COMPILE_TEST,
so we run into a harmless randconfig warning on ARM when CONFIG_PM
is disabled:

drivers/input/touchscreen/migor_ts.c:216:12: error: 'migor_ts_resume' defined but not used [-Werror=unused-function]
drivers/input/touchscreen/migor_ts.c:205:12: error: 'migor_ts_suspend' defined but not used [-Werror=unused-function]

This shuts up the warning by marking the two functions as __maybe_unused.
We could do the same thing by adding an #ifdef, but this version is
more reliable.

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