Introduce new generic alloc and free methods with attributes argument.

Existing alloc_coherent and free_coherent can be implemented on top of the
new calls with NULL attributes argument. Later also dma_alloc_non_coherent
can be implemented using DMA_ATTR_NONCOHERENT attribute as well as
dma_alloc_writecombine with separate DMA_ATTR_WRITECOMBINE attribute.

This way the drivers will get more generic, platform independent way of
allocating dma buffers with specific parameters.
Signed-off-by: NMarek Szyprowski <m.szyprowski@samsung.com>
Acked-by: NKyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: NDavid Gibson <david@gibson.dropbear.ud.au>
Reviewed-by: NArnd Bergmann <arnd@arndb.de>