lp8788-ldo: fix a parent device on devm_gpio_request()

 Use 'platform_device' rather than i2c client device node.
 Argument is added in lp8788_config_ldo_enable_mode().

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>