mfd: rk808: Use devm_mfd_add_devices() for mfd_device registration

Use devm_mfd_add_devices() for MFD devices registration and remove
the call of mfd_remove_devices() from .remove callback to remove
MFD child-devices. This is done by managed device framework.

CC: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>