MIPS noncoherent DMA cleanups v2

[Christoph Hellwig]

Hi Thomas,

this series cleans up some of the mips (maybe) noncoherent support.
It also remove the need for the special <asm/dma-coherence.h> header only
provided by mips.

Changes since v1:
 - fix a bisection issue due to a missing brace
 - simplify the parameter parsing given that it happens after
   plat_mem_init