mfd: stw481x: 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.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>