MIPS: BMIPS: Move post DMA flush implementation to common header
arch/mips/include/asm/mach-bmips/dma-coherence.h contains the
plat_post_dma_flush implementation which is not specific to mach-bmips,
but required for all BMIPS-based systems.
Move plat_post_dma_flush to arch/mips/include/asm/bmips.h, rename it to
bmips_post_dma_flush such that other platforms like bcm63xx can utilize
it.