mtd: nand: pxa3xx: set mtd->dev

Currently the pxa3xx driver does not set the udevice in the mtd_info
struct and this prevents the mtd from parsing the partitions via DTS
like for SPI-NOR.

So simply set the mtd->dev to the driver udevice.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Reviewed-by: Stefan Roese <sr@denx.de>