ARM/dma-mapping: remove the dma_coherent member of struct dev_archdata

Since commit ae626eb9 ("ARM/dma-mapping: use dma-direct
unconditionally") only the dma_coherent flag in struct device is used,
so remove the now write only flag in struct dev_archdata.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>