bus: ti-sysc: mark PM functions as __maybe_unused

The new bus driver causes a harmless compile-time warning when
CONFIG_PM is disabled:

drivers/bus/ti-sysc.c:440:12: error: 'sysc_runtime_resume' defined but not used [-Werror=unused-function]
 static int sysc_runtime_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~
drivers/bus/ti-sysc.c:421:12: error: 'sysc_runtime_suspend' defined but not used [-Werror=unused-function]
 static int sysc_runtime_suspend(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~~

This marks the two unused functions as __maybe_unused to shut up
that warning.

Fixes: 0eecc636 ("bus: ti-sysc: Add minimal TI sysc interconnect target driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>