led: Configure LED default-state on boot

In case the DT LED subnode contains "default-state" property set to
either "on" or "off", probe the LED driver and configure the LED state
automatically.

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>
[trini: Update the relevant test now that we have support]
Signed-off-by: Tom Rini <trini@konsulko.com>