devicectree: bindings: fix location of leds common file

The leds.txt was moved and renamed. Fix references to
it accordingly.

Fixes: f6760539 ("devicetree/bindings: Move gpio-leds binding into leds directory")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>