soc: nordic: pinctrl: rework nordic,clock-enable

Instead of forcing users to provide this setting, allow to describe
which signals require CLOCKPIN enablement at device nodes. This is later
captured by the pinctrl macros and applied in the pinctrl driver. Note
that name has been adjusted to nordic,clockpin-enable to avoid confusion
with clock related settings.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>