dts: bindings: nordic: Require pinctrl-names together with pinctrl-0

... so that a clear devicetree error is reported when the pinctrl-names
property is missing, not a quite cryptic compilation error about an
undeclared PINCTRL_STATE_*_UPPER_TOKEN symbol in pinctrl.h.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>