[media] v4l: rcar_fdp1: mark PM functions as __maybe_unused

The new driver produces a warning when CONFIG_PM is disabled:

platform/rcar_fdp1.c:2408:12: error: 'fdp1_pm_runtime_resume' defined but not used [-Werror=unused-function]
platform/rcar_fdp1.c:2399:12: error: 'fdp1_pm_runtime_suspend' defined but not used [-Werror=unused-function]

This marks the two functions as __maybe_unused.

Fixes: 4710b752 ("[media] v4l: Add Renesas R-Car FDP1 Driver")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>