arm64: dts: freescale: sl28: map GPIOs to input events

Now that we have support for GPIO lines of the SMARC connector, map the
sleep, power and lid switch signals to the corresponding keys using the
gpio-keys and gpio-keys-polled drivers. The power and sleep signals have
dedicated interrupts, thus we use these ones. The lid switch is just
mapped to a GPIO input and needs polling.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>