mfd: mfd-core: Move pdev->mfd_cell creation back into mfd_add_device()

Most of the complexity of mfd_platform_add_cell() has been removed. The
only functionality left duplicates cell memory into the child's platform
device. Since it's only a few lines, moving it to the main thread and
removing the superfluous function makes sense.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Mark Brown <broonie@kernel.org>