pinctrl: sh-pfc: Rollback to mux if required when the gpio is freed

Some drivers require switching between function and gpio at run-time.
Allow to roll back from gpio to mux when the gpio is freed.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>