MIPS: BCM63xx: Provide a plat_post_dma_flush hook

MIPS: BCM63xx: Provide a plat_post_dma_flush hook

Broadcom BCM63xx DSL SoCs utilize BMIPS CPUs, and as such are required
to perform a read-ahead cache flush after a DMA transfer. Utilize
asm/bmips.h to provide a plat_post_dma_flush_hook, and
mach-generic/dma-coherence.h for everything else.

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/9726/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>