mtd: nand: pxa3xx_nand: fix dmaengine initialization

When the driver is initialized in a pure device-tree platform, the
driver's probe fails allocating the dma channel :
[  525.624435] pxa3xx-nand 43100000.nand: no resource defined for data DMA
[  525.632088] pxa3xx-nand 43100000.nand: alloc nand resource failed

The reason is that the DMA IO resource is not acquired through platform
resources but by OF bindings.

Fix this by ensuring that DMA IO resources are only queried in the non
device-tree case.

Fixes: 8f5ba31a ("mtd: nand: pxa3xx-nand: switch to dmaengine")
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>