blackfin, mips, openrisc, ppc, sandbox, x86: add generic dma_alloc, dma_free inlines ...

Some drivers call dma_inv_range() on buffers, on arm these buffers must
be cache line aligned. This patch introduces a generic dma_alloc,
dma_free. Archs can implement in their own functions in "asm/dma.h" and add a:

	#define dma_alloc dma_alloc
	#define dma_free dma_free

On all other archs the generic versions, which translate into xmalloc
and free are used.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>