ARM i.MX dts: Consistently add labels to devicenodes

Having labels before each node allows board bindings to reference
to nodes by using the &nodename {} notation. This way boards do not
have to resemble the whole devicetree layout. Due to less indention
needed the board files also get better readability. Since the label
make the documentation behind the nodes unnecessary it is removed.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>