dma-mapping: Provide write-combine allocations

Provide an implementation for dma_{alloc,free,mmap}_writecombine() when
the architecture supports DMA attributes.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>