ARM: dts: at91: smartkiz: Reference led node directly

Do it the simple way like for the other kizbox boards.  This will allow
renaming the led controller node name later without breaking things.

Signed-off-by: Alexander Dahl <post@lespocky.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201005203451.9985-5-post@lespocky.de