powerpc/ps3: Fix CONFIG_PS3_FLASH=n build warning

Fix build warnings like these when CONFIG_PS3_FLASH=n:

  arch/powerpc/platforms/ps3/os-area.c: warning: 'update_flash_db' defined but not used
Signed-off-by: NGeoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/platforms/ps3/os-area.c

@@ -578,7 +578,7 @@ static void os_area_db_init(struct os_area_db *db)
  *
  */
 
-static void update_flash_db(void)
+static void __maybe_unused update_flash_db(void)
 {
 	int result;
 	int file;