dt-bindings: leds: Convert gpio-leds to DT schema

Convert the gpio-leds binding to DT schema format.

Drop the last example as the node name collides when built, and it doesn't
add much value.

Cc: Pavel Machek <pavel@ucw.cz>
Cc: Dan Murphy <dmurphy@ti.com>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>