AM64 SoC contains the GPMC NAND controller. Add compatible for it.

Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Roger Quadros <rogerq@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211209090458.24830-2-rogerq@kernel.org