dt-bindings: arm: fsl: Add Seeed studio NPi based boards

NPi i.MX6ULL eMMC/NAND is Seed Studios SoM using i.MX6ULL.
Development baords can be either based on NAND or eMMC
SoM.

Signed-off-by: Parthiban Nallathambi <parthiban@linumiz.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Author: parthitce

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -813,6 +813,14 @@ properties:
           - const: tq,imx6ull-tqma6ull2l      # MCIMX6Y2, LGA SoM variant
           - const: fsl,imx6ull
 
+      - description: Seeed Stuido i.MX6ULL SoM on dev boards
+        items:
+          - enum:
+              - seeed,imx6ull-seeed-npi-emmc
+              - seeed,imx6ull-seeed-npi-nand
+          - const: seeed,imx6ull-seeed-npi
+          - const: fsl,imx6ull
+
       - description: i.MX6ULZ based Boards
         items:
           - enum: