dt-bindings: arm: imx: add imx8qxp mek support

i.MX 8QuadXPlus is a quad (4x) Cortex-A35 proccessor with powerful
graphic and multimedia features.
This patch adds imx8qxp mek board support.

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>