MIPS: DMA: Allow platforms to override only the post DMA hook

MIPS: DMA: Allow platforms to override only the post DMA hook

Instead of having platforms to copy the entirety of
mach-generic/dma-coherence.h, check whether these platforms have already
defined a plat_post_dma_flush hook, and if not, provide an inline stub.

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/9725/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>