mfd: wm8400-core: Make mfd_cell array const

mfd_add_devices() expects array of struct mfd_cell to be const.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>