dt-bindings: mtd: Add AC5 specific binding

Add binding for AC5 SoC. This SoC only supports NAND SDR timings up to
mode 3 so a specific compatible value is needed.

Signed-off-by: Chris Packham <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>
Link: https://lore.kernel.org/linux-mtd/[e-mail]

Documentation/devicetree/bindings/mtd/marvell,nand-controller.yaml

···
           - const: marvell,armada-8k-nand-controller
           - const: marvell,armada370-nand-controller
       - enum:
+          - marvell,ac5-nand-controller
           - marvell,armada370-nand-controller
           - marvell,pxa3xx-nand-controller
       - description: legacy bindings