move dma_ops to common code

[Antony Pavlov]

Hi!

In 0e885ce81d0e ('RISC-V: dma: support multiple dma_alloc_coherent backends') ,
(see https://lore.barebox.org/barebox/20210619045055.779-10-a.fatoum@pengutronix.de/)
multiple dma_alloc_coherent backends was introduced for RISC-V.

At the moment MIPS dma_alloc_coherent stuff is messy and I want to rework it.
I can reuse some parts of 0e885ce81d0e by moving it to the common code.
Any comments or suggestions?

-- 
Best regards,
  Antony Pavlov