mfd: Standardise MFD_CELL_* helper names

Start all helpers with "MFD_CELL_".

Cc: Gene Chen <gene_chen@richtek.com>
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>