mfd: Constify struct mfd_cell where possible

mfd: Constify struct mfd_cell where possible

As of commit 30608690597d03087e454a47a7c40a56e8a4b997 ("mfd: Stop setting
refcounting pointers in original mfd_cell arrays"), the "cell" parameter of
mfd_add_devices() is "const" again. Hence make all cell data passed to
mfd_add_devices() const where possible.

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