dt-bindings: leds: Add pattern initialization from Device Tree

Document new led-pattern property for initialization of LED triggers.
The property format is trigger-specific (except being array of
integers).  For pattern trigger, the explanation of pattern format was
moved to a common file shared with sysfs ABI.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>