i2c: mux: demux-pinctrl: Update docs to new sysfs-attributes

Update the docs according to the recent code changes, too.

Fixes: c0c508a4 ("i2c: mux: demux-pinctrl: Clean up sysfs attributes")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>