mfd: mfd-core: Honour Device Tree's request to disable a child-device

Until now, MFD has assumed all child devices passed to it (via
mfd_cells) are to be registered. It does not take into account
requests from Device Tree and the like to disable child devices
on a per-platform basis.

Well now it does.

Link: https://www.spinics.net/lists/arm-kernel/msg366309.html
Link: https://lkml.org/lkml/2019/8/22/1350



Reported-by: Barry Song <Baohua.Song@csr.com>
Reported-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>