dt-bindings: mtd: gpmi-nand: Add compatible string for i.MX8 chips

Add compatible string "fsl,imx8mp-gpmi-nand" and "fsl,imx8mq-gpmi-nand",
which back compatible with i.MX7D. So set these fall back to
"fsl,imx7d-gpmi-nand".

Add compatible string "fsl,imx8qm-gpmi-nand" and "fsl,imx8dxl-gpmi-nand",
which back compatible with i.MX8QXP. So set these fall back to
"fsl,imx8qxp-gpmi-nand".

Signed-off-by: Frank Li <[e-mail]>
Reviewed-by: Han Xu <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>

Documentation/devicetree/bindings/mtd/gpmi-nand.yaml

···
           - enum:
               - fsl,imx8mm-gpmi-nand
               - fsl,imx8mn-gpmi-nand
+              - fsl,imx8mp-gpmi-nand
+              - fsl,imx8mq-gpmi-nand
           - const: fsl,imx7d-gpmi-nand
+      - items:
+          - enum:
+              - fsl,imx8dxl-gpmi-nand
+              - fsl,imx8qm-gpmi-nand
+          - const: fsl,imx8qxp-gpmi-nand
 
   reg:
     items: