drm/panfrost: Add PM runtime flag

drm/panfrost: Add PM runtime flag

When the GPU is the only device attached to a single power domain,
core genpd disable and enable it when gpu enter and leave runtime suspend.

Some power-domain requires a sequence before disabled,
and the reverse when enabled.

Add GPU_PM_RT flag, and implement in
panfrost_device_runtime_suspend/resume.

Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Philippe Simons <simons.philippe@gmail.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20250403055210.54486-2-simons.philippe@gmail.com