drm/nouveau/pwr: assign a subdev id for upcoming implementation

THERM will use this before it exists.
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/core/include/core/device.h

@@ -33,6 +33,7 @@ enum nv_subdev_type {
 	NVDEV_SUBDEV_INSTMEM,
 	NVDEV_SUBDEV_VM,
 	NVDEV_SUBDEV_BAR,
+	NVDEV_SUBDEV_PWR,
 	NVDEV_SUBDEV_VOLT,
 	NVDEV_SUBDEV_CLOCK,
 	NVDEV_SUBDEV_THERM,