mfd: wm8400: Use devm_mfd_add_devices() for mfd_device registration
Use devm_mfd_add_devices() for MFD devices registration and get
rid of .remove callback to remove MFD child-devices. This is done
by managed device framework.
CC: Mark Brown <broonie@kernel.org> Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com> Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com> Signed-off-by: Lee Jones <lee.jones@linaro.org>