mfd: cs5535-mfd: Register clients using their own dedicated MFD cell entries

CS5535 is the only user of mfd_clone_cell().  It makes more sense to
register child devices in the traditional way and remove the quite
bespoke mfd_clone_cell() call from the MFD API.

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