DT: leds: Improve description of flash LEDs related properties

1. Since max-microamp property has had no users so far, then rename
   it to more descriptive led-max-microamp.
2. Since flash-timeout-us property has had no users so far, then rename
   it to more accurate flash-max-timeout-us.
3. Describe led-max-microamp property as mandatory for specific board
   configurations.
4. Make flash-max-microamp and flash-max-timeout-us properties mandatory
   for devices with configurable flash current and flash timeout settings
   respectively.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: devicetree@vger.kernel.org
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>