leds: core: add OF variants of LED registering functions

These new functions allow passing an additional device_node argument
that will be internally set for created LED device. Thanks to this LED
core code and triggers will be able to access DT node for reading extra
info.

The easiest solution for achieving this was reworking old functions to
more generic ones & adding simple defines for API compatibility.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>