PCI: rockchip: Mark PM functions as __maybe_unused

When CONFIG_PM_SLEEP is disabled, we get harmless build warnings:

  host/pcie-rockchip.c:1267:12: error: 'rockchip_pcie_resume_noirq' defined but not used [-Werror=unused-function]
  host/pcie-rockchip.c:1240:12: error: 'rockchip_pcie_suspend_noirq' defined but not used [-Werror=unused-function]

Marking both functions as __maybe_unused avoids the warning without the
need for #ifdef around them.

Fixes: 013dd3d5 ("PCI: rockchip: Add system PM support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Shawn Lin <shawn.lin@rock-chips.com>