mtd: pxa3xx_nand: port to use driver model

Use the generic DT code to find the device compatible property for us.
This makes the driver look more like other current drivers. It also make
it easier to add support for other variants like Armada 8K in a future
commit.

Signed-off-by: Shmuel Hazan <shmuel.h@siklu.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>