leds: Add support for max77693 mfd flash cell

This patch adds led-flash support to Maxim max77693 chipset.
A device can be exposed to user space through LED subsystem
sysfs interface. Device supports up to two leds which can
work in flash and torch mode. The leds can be triggered
externally or by software.

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>
Cc: Richard Purdie <rpurdie@rpsys.net>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>