regulator: lp8788-ldo: use ena_pin of regulator-core for external control

 Regulator core driver provides enable GPIO control for enabling/disabling a
 regulator. Now, enable GPIO is shared among regulators.
 Use this internal working, so unnecessary code are removed.
 GPIO enable pin configurations are added in digital LDO and analog LDO drivers.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Reviewed-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>