mfd: max77693: Modify flash cell name identifiers

Change flash cell identifiers from max77693-flash to max77693-led
to avoid confusion with NOR/NAND Flash.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>