ARM: dts: mxs: Split M28EVK into SoM and EVK parts

This patch splits the M28EVK device tree file into a common SoM
part and an EVK part. This is needed to make it easier for users
of the SoM to put it into different, non-reference baseboard.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>