mfd: max77693: Improve support for the flash cell

This patch improves support for the flash cell of
max77693 mfd by adding relevant of_compatible field
and a structure for caching related platform data.
Added are also FLASH registers related macro definitions.

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