arm64: dts: imx8m*-tqma8m*: Add chassis-type

Device tree specification 0.4 defines an optional, but recommended
'chassis-type' property. Add it to TQMa8M* based board files.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>