staging: greybus: mark PM functions as __maybe_unused

Enabling the arche platform for compile testing showed a harmless
warning with CONFIG_PM=n:

drivers/staging/greybus/arche-platform.c:632:12: error: 'arche_platform_resume' defined but not used [-Werror=unused-function]
drivers/staging/greybus/arche-platform.c:618:12: error: 'arche_platform_suspend' defined but not used [-Werror=unused-function]

This marks the functions as __maybe_unused to shut up the warnings.

Fixes: 2eccd4aa19 ("staging: greybus: enable compile testing of arche driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/greybus/arche-platform.c

@@ -615,7 +615,7 @@ static int arche_platform_remove(struct platform_device *pdev)
 	return 0;
 }
 
-static int arche_platform_suspend(struct device *dev)
+static __maybe_unused int arche_platform_suspend(struct device *dev)
 {
 	/*
 	 * If timing profile premits, we may shutdown bridge
@@ -629,7 +629,7 @@ static int arche_platform_suspend(struct device *dev)
 	return 0;
 }
 
-static int arche_platform_resume(struct device *dev)
+static __maybe_unused int arche_platform_resume(struct device *dev)
 {
 	/*
 	 * Atleast for ES2 we have to meet the delay requirement between