Seeing doubtfully any SATA device working without embedded DMA engine
let's permit the device nodes being equipped with the dma-coherent
property in case if the platform is capable of cache-coherent DMAs.

As a side-effect we can drop the explicit dma-coherent property definition
from the particular device schemas. Currently it concerns the Broadcom
SATA AHCI controller only.
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>