mfd: Constify struct mfd_cell where possible

mfd: Constify struct mfd_cell where possible

As of commit 82e54b28ae65f7fd69bfc21ae3eea8715998fadc ("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>