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 <chris.packham@alliedtelesis.co.nz>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230703035044.2063303-2-chris.packham@alliedtelesis.co.nz