clk: mmp2: fix build without CONFIG_PM

clk: mmp2: fix build without CONFIG_PM

pm_clk_suspend()/pm_clk_resume() are defined as NULL pointers rather than
empty inline stubs without CONFIG_PM:

drivers/clk/mmp/clk-audio.c:402:16: error: called object type 'void *' is not a function or function pointer
        pm_clk_suspend(dev);
drivers/clk/mmp/clk-audio.c:411:15: error: called object type 'void *' is not a function or function pointer
        pm_clk_resume(dev);

I tried redefining the helper functions, but that caused additional
problems. This is the simple solution of replacing the __maybe_unused
trick with an #ifdef.

Fixes: 725262d29139 ("clk: mmp2: Add audio clock controller driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210103135503.3668784-1-arnd@kernel.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>