mtd: nand: pxa3xx: devicetree binding update

Since the driver supports the new compatible string, the binding
documentation must be updated to reflect it.

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>