mfd: mfd-core: Add mechanism for removal of a subset of children

mfd: mfd-core: Add mechanism for removal of a subset of children

Currently, the only way to remove MFD children is with a call to
mfd_remove_devices, which will remove all the children. Under
some circumstances it is useful to remove only a subset of the
child devices. For example if some additional clean up is required
between removal of certain child devices.

To accomplish this a level field is added to mfd_cell, the normal
mfd_remove_devices is modified to not remove devices that are set
to a higher level and a corresponding mfd_remove_devices_late
function is added to remove those children.

See further discussion at:
https://lore.kernel.org/lkml/20200616075834.GF2608702@dell/

Suggested-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>