lp8788-buck: fix a parent deivce in _probe()

 The lp8788-buck is a platform driver of lp8788-mfd.
 The platform device is allocated when mfd_add_devices() is called
 in lp8788-mfd.
 On the other hand, 'lp->dev' is the i2c client device.

 Therefore, this 'platform_device' is a proper parent device in case of
 resource managed mem alloc, registering a regulator and device kernel message.

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