mtd: nand: pxa3xx: Enable devbus/nand arbiter on Armada 8K

The CN9130 SoC (an ARMADA 8K type) has both a NAND Flash Controller and
a generic local bus controller (Device Bus Controller) that share common
pins.

With a board design that incorporates both a NAND flash and uses
the Device Bus (in our case for an SRAM) accessing the Device Bus device
fails unless the NfArbiterEn bit is set. Setting the bit enables
arbitration between the Device Bus and the NAND flash.

Since there is no obvious downside in enabling this for designs that
don't require arbitration, we always enable it.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>