i2c: mux: demux-pinctrl: run properly with multiple instances

We can't use a static property for all the changesets, so we now create
dynamic ones for each changeset.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Fixes: 50a5ba87 ("i2c: mux: demux-pinctrl: add driver")
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>