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 <[e-mail]>
Cc: Richard Purdie <[e-mail]>
Cc: Pavel Machek <[e-mail]>
Cc: [e-mail]
Acked-by: Sakari Ailus <[e-mail]>
Acked-by: Sylwester Nawrocki <[e-mail]>

Documentation/devicetree/bindings/leds/common.txt

···
      "ide-disk" - LED indicates disk activity
      "timer" - LED flashes at a fixed, configurable rate
 
-- max-microamp : maximum intensity in microamperes of the LED
-		 (torch LED for flash devices)
-- flash-max-microamp : maximum intensity in microamperes of the
-                       flash LED; it is mandatory if the LED should
-		       support the flash mode
-- flash-timeout-us : timeout in microseconds after which the flash
-                     LED is turned off
+- led-max-microamp : Maximum LED supply current in microamperes. This property
+                     can be made mandatory for the board configurations
+                     introducing a risk of hardware damage in case an excessive
+                     current is set.
+                     For flash LED controllers with configurable current this
+                     property is mandatory for the LEDs in the non-flash modes
+                     (e.g. torch or indicator).
 
+Required properties for flash LED child nodes:
+- flash-max-microamp : Maximum flash LED supply current in microamperes.
+- flash-max-timeout-us : Maximum timeout in microseconds after which the flash
+                         LED is turned off.
+
+For controllers that have no configurable current the flash-max-microamp
+property can be omitted.
+For controllers that have no configurable timeout the flash-max-timeout-us
+property can be omitted.
 
 Examples:
 
···
 camera-flash {
 	label = "Flash";
 	led-sources = <0>, <1>;
-	max-microamp = <50000>;
+	led-max-microamp = <50000>;
 	flash-max-microamp = <320000>;
-	flash-timeout-us = <500000>;
+	flash-max-timeout-us = <500000>;
 };