s390: Implement dma_{alloc,free}_attrs()

The S390 architecture advertises support for HAVE_DMA_ATTRS when PCI is
enabled. Patches to unify some of the DMA API would like to rely on the
dma_alloc_attrs() and dma_free_attrs() functions to be provided when an
architecture supports DMA attributes.

Rename dma_alloc_coherent() and dma_free_coherent() to dma_alloc_attrs()
and dma_free_attrs() since they are functionally equivalent and alias
the former to the latter for compatibility.

For consistency with other architectures, also reuse the existing symbol
HAVE_DMA_ATTRS defined in arch/Kconfig instead of providing a duplicate.
Select it when PCI is enabled.

While at it, drop a redundant 'default n' from the PCI Kconfig symbol.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-By: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>