phy: tegra: xusb: mark PM functions as __maybe_unused

When these are not referenced, gcc prints a harmless warning:

drivers/phy/tegra/xusb.c:1286:12: error: 'tegra_xusb_padctl_resume_noirq' defined but not used [-Werror=unused-function]
 1286 | static int tegra_xusb_padctl_resume_noirq(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/phy/tegra/xusb.c:1276:12: error: 'tegra_xusb_padctl_suspend_noirq' defined but not used [-Werror=unused-function]
 1276 | static int tegra_xusb_padctl_suspend_noirq(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: c545a905

 ("phy: tegra: xusb: Add sleepwalk and suspend/resume")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210721152550.2976003-1-arnd@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>