soc: ti: wkup_m3_ipc: mark PM functions as __maybe_unused

Everyone gets these #ifdefs wrong, leading to another warning here:

drivers/soc/ti/wkup_m3_ipc.c:547:12: error: 'wkup_m3_ipc_resume' defined but not used [-Werror=unused-function]
 static int wkup_m3_ipc_resume(struct device *dev)
drivers/soc/ti/wkup_m3_ipc.c:539:12: error: 'wkup_m3_ipc_suspend' defined but not used [-Werror=unused-function]
 static int wkup_m3_ipc_suspend(struct device *dev)

The easiest way to get it right is to remove all the #ifdefs and
let the compiler drop the unused functions silently after we
mark them as __maybe_unused.

Fixes: 7a872b6f ("soc: ti: wkup_m3_ipc: Add rtc_only with ddr in self refresh mode support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>