drivers: pinctrl: allow to skip states

If a certain state has to be skipped, a macro named
Z_PINCTRL_SKIP_<STATE> can be defined evaluating to 1. This can be
useful, for example, to automatically ignore the sleep state if no
device power management is enabled.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>