regulator: gpio-regulator: fix can't find regulator node in dt

Need initilize of_node in regulator config when register regulator,
otherwise regulator driver think it is no-dt device.

in regulator_dev_lookup
list_for_each_entry(r, &regulator_list, list)
	if (r->dev.parent &&
		node == r->dev.of_node)
			return r

r->dev.of_noe will be zero if miss config in cfg.

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>