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 <Frank.Li@nxp.com>
Reviewed-by: Han Xu <han.xu@nxp.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>