pinctrl: stm32: bind only the enabled GPIO subnode

pinctrl: stm32: bind only the enabled GPIO subnode

Bind only the enabled GPIO subnode, to avoid to probe the node
"gpio-controller" present in SOC dtsi (disabled by default) but
not enabled in the included pincontrol dtsi file.

For example, in stm32mp15xxac-pinctrl.dtsi 2 gpio bank are absent:
 gpioj: gpio@5000b000
 gpiok: gpio@5000c000

Then these GPIO are absent in output of command "dm tree" and
"gpio status -a"

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>