mtd: nand: pxa3xx_nand: use nand_to_mtd()

Don't store struct mtd_info in struct pxa3xx_nand_host. Instead use the
one that is already part of struct nand_chip. This brings us in line
with current U-boot and Linux conventions.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Ofer Heifetz <oferh@marvell.com>
Reviewed-by: Igal Liberman <igall@marvell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Stefan Roese <sr@denx.de>