arm64: dts: freescale: Use overlay target for simplicity

With commit 15d16d6d ("kbuild: Add generic rule to apply
fdtoverlay"), overlay target can be used to simplify the build of DTB
overlays.  It also performs a cross check to ensure base DT and overlay
actually match.

Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>