phy: dm816x: use __maybe_unused to hide pm functions

The dm816x USB PHY driver uses UNIVERSAL_DEV_PM_OPS to access
its suspend/resume functions, which causes a warning about
unused symbols when CONFIG_PM is disabled:

drivers/phy/phy-dm816x-usb.c:121:12: error: 'dm816x_usb_phy_runtime_suspend' defined but not used [-Werror=unused-function]
drivers/phy/phy-dm816x-usb.c:139:12: error: 'dm816x_usb_phy_runtime_resume' defined but not used [-Werror=unused-function]

This adds __maybe_unused annotations to let the compiler know
it can silently drop the function definition.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>