base-files: unify get_dt_led helper function

base-files: unify get_dt_led helper function

Lantiq and IPQ806X (which includes IPQ40XX) both define the
same custom function {ipq806x|lantiq}_get_dt_led.

This patch moves the function into the base-file package at
lib/functions/leds.sh to make it more accessible for other
targets as well.

Cc: Mathias Kresin <dev@kresin.me>
Cc: John Crispin <john@phrozen.org>
Cc: Hannu Nyman <hannu.nyman@iki.fi>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>