extres/regulator: Properly refcount gpio regulators

regnode::enable_cnt is generally used to refcount regulator nodes. For
GPIOs, the refcount was done on the gpio_entry since more than one regulator
can share a GPIO.

GPIO regulators were not taking part in the node refcount, since they had
their own mechanism. This caused some fallout after manu started disabling
everybody's unused regulators in r331989.

Refcount it.

Glanced over by: manu