dts: bindings: pinctrl: nrf: add nordic,clock-enable

On new SoCs, certain pins need to enable the clock setting on the pin
for it to work properly. For now, this has been handled internally in
the pinctrl driver, however, it appears to be an instance-specific
property (e.g. UARTE/SPIM instances in the fast domain do not require
such setting). Move the configuration of this settings to DT, the best
place where to have instance-specific hardware settings.

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