arm64: dts: freescale: sl28: enable LED support

Now that we have support for GPIO lines of the SMARC connector, enable
LED support on the KBox A-230-LS. There are two LEDs without fixed
functions, one is yellow and one is green. Unfortunately, it is just one
multi-color LED, thus while it is possible to enable both at the same
time it is hard to tell the difference between "yellow only" and "yellow
and green".

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>