mtd: nand: pxa3xx: Add "armada370-nand" compatible

Now that the driver can support the Armada 370/XP SoC NAND controller,
add the devicetree compatible string, enabling its use.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>