led: add a struct led_dt_spec and some _dt wrapper APIs

Add a struct led_dt_spec to hold an LED device and index pointer, some
initializer and wrapper APIs.

This allows simpler LED usage, such as:

static const struct led_dt_spec led = LED_DT_SPEC_GET(DT_NODELABEL(led0));
led_on_dt(&led);
led_off_dt(&led);

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>