mfd: Stop setting refcounting pointers in original mfd_cell arrays

Commit 1e29af62 ("mfd: Add refcounting
support to mfd_cells") had to drop the "const" keyword on the "cell"
parameter of mfd_add_devices(), as it added the refcounting pointers
to the objects of the passed mfd_cell array itself.

However, the mfd core code operates on copies of the mfd_cell objects,
so there's no need to modify the originally passed objects.

Hence, move the setting of the refcounting pointers from mfd_add_devices()
to mfd_platform_add_cell(), where the copy of the mfd_cell objects is made.
mfd_clone_cell() can just pass (a copy of) the original usage_count
pointer.

This allows to make the "cell" parameter of mfd_add_devices() "const"
again, and avoids future race conditions when registering multiple
instances of the same device in parallel.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>