dt-bindings: arm: fsl: Add binding for imx8ulp evk

Add the dt binding for i.MX8ULP EVK board.

i.MX 8ULP is part of the ULP family with emphasis on extreme
low-power techniques using the 28 nm fully depleted silicon on
insulator process. Like i.MX 7ULP, i.MX 8ULP continues to be
based on asymmetric architecture, however will add a third DSP
domain for advanced voice/audio capability and a Graphics domain
where it is possible to access graphics resources from the
application side or the realtime side.
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -872,6 +872,12 @@ properties:
           - const: toradex,colibri-imx8x
           - const: fsl,imx8qxp
 
+      - description: i.MX8ULP based Boards
+        items:
+          - enum:
+              - fsl,imx8ulp-evk           # i.MX8ULP EVK Board
+          - const: fsl,imx8ulp
+
       - description:
           Freescale Vybrid Platform Device Tree Bindings