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.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: cernekee@gmail.com
Cc: jogo@openwrt.org
Patchwork: https://patchwork.linux-mips.org/patch/9724/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>