phy: zynqmp: Fix unused-function compiler warning

phy: zynqmp: Fix unused-function compiler warning

Mark xpsgtr_suspend and xpsgtr_resume as __maybe_unused to fix the
following compiler warning when building with !CONFIG_PM_SLEEP:

drivers/phy/xilinx/phy-zynqmp.c:830:12: warning: ‘xpsgtr_resume’ defined but not used [-Wunused-function]
  830 | static int xpsgtr_resume(struct device *dev)
      |            ^~~~~~~~~~~~~
drivers/phy/xilinx/phy-zynqmp.c:819:12: warning: ‘xpsgtr_suspend’ defined but not used [-Wunused-function]
  819 | static int xpsgtr_suspend(struct device *dev)
      |            ^~~~~~~~~~~~~~

Also drop the existing #ifdef CONFIG_PM so the functions are always
compile-checked regardless of CONFIG_PM and/or CONFIG_PM_SLEEP being
set.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Link: https://lore.kernel.org/r/20200701141017.26931-1-tklauser@distanz.ch
Signed-off-by: Vinod Koul <vkoul@kernel.org>