leds: add device tree bindings for register bit LEDs

This adds the device tree bindings used by register bit LEDs.

Cc: devicetree@vger.kernel.org
Cc: Bryan Wu <cooloney@gmail.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>