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

 Use 'platform_device' rather than i2c client device node.
 Arguments are added in lp8788_init_dvs() and lp8788_dvs_gpio_request().

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