ARM: dts: imx28-apx4devkit: Convert to use label references

This 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 orer to make it
easier to maintain.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Cc: Aapo Vienamo <aapo.vienamo@iki.fi>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>