led: Move OF "label" property parsing to core

Every driver in drivers/led/ currently does some form of "label" OF
property parsing in its bind() callback. Move this label parsing to
LED core, since this "label" OF property is a generic property. This
permits code deduplication in subseuqent patches.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alex Nemirovsky <alex.nemirovsky@cortina-access.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Cc: Philippe Reynes <philippe.reynes@softathome.com>
Cc: Sean Anderson <seanga2@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Steven Lawrance <steven.lawrance@softathome.com>