dt-bindings: arm: Move Emtrion i.MX6 board bindings to schema

The Emtrion board bindings landed when the i.MX board/SoC bindings were
being converted to DT schema. Add them to the schema and remove the
separate file.

Cc: Jan Tuerk <jan.tuerk@emtrion.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>