mfd: intel-lpss: Hide suspend/resume functions in #ifdef

mfd: intel-lpss: Hide suspend/resume functions in #ifdef

When CONFIG_PM is disabled, the prototypes for these two functions
are not visible:

drivers/mfd/intel-lpss.c:482:5: error: no previous prototype for 'intel_lpss_suspend' [-Werror=missing-prototypes]
  482 | int intel_lpss_suspend(struct device *dev)
      |     ^~~~~~~~~~~~~~~~~~
drivers/mfd/intel-lpss.c:503:5: error: no previous prototype for 'intel_lpss_resume' [-Werror=missing-prototypes]
  503 | int intel_lpss_resume(struct device *dev)

Add the same #ifdef around the definition of the unused functions,
which avoids the warning and slightly reduces the code size.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230516202746.561111-1-arnd@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>