mfd: kempld-core: Make mfd_cell array const

The kempld_devs (array of mfd_cells) can be safely made const.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Kevin Strasser <kevin.strasser@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>