i2c: mux: demux-pinctrl: disable PM user interface

The demux device is only a logical device with no children. So, no
RuntimePM is needed, let's disable the sysfs interface for it.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Peter Rosin <peda@axentia.se>