usb: cdns3: imx: mark cdns_imx_system_resume as __maybe_unused

The function cdns_imx_system_resume() may have no callers depending
on configuration, so it must be marked __maybe_unused to avoid
harmless warning:

drivers/usb/cdns3/cdns3-imx.c:378:12: warning:
 'cdns_imx_system_resume' defined but not used [-Wunused-function]
  378 | static int cdns_imx_system_resume(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~~~~~~

Fixes: 67982dfa59de ("usb: cdns3: imx: add power lost support for system resume")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Peter Chen <peter.chen@kernel.org>