ARM: dts: imx28-evk: Convert to use label references

This Freescale board file still use node name and unit address
to reference parts from the imx28.dtsi . This causes a lot of
redundancy. So use label references in order to make it easier
to maintain.
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>