dt-bindings: mtd: remove pxa3xx NAND controller documentation

The deprecated pxa3xx_nand.c driver does not exist anymore, it has been
replaced by marvell_nand.c which has its own up-to-date documentation.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Tested-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>