mfd: mfd-core: Remove mfd_clone_cell()

Providing a subsystem-level API helper seems over-kill just to save a
few lines of C-code.  Previous commits saw us convert mfd_clone_cell()'s
only user over to use a more traditional style of MFD child-device
registration.  Now we can remove the superfluous helper from the MFD API.

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