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>

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -151,6 +151,12 @@ properties:
           - const: compulab,cl-som-imx7
           - const: fsl,imx7d
 
+      - description: i.MX8QXP based Boards
+        items:
+          - enum:
+              - fsl,imx8qxp-mek           # i.MX8QXP MEK Board
+          - const: fsl,imx8qxp
+
       - description:
           Freescale Vybrid Platform Device Tree Bindings