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>

arch/mips/include/asm/mach-bcm63xx/dma-coherence.h

+#ifndef __ASM_MACH_BCM63XX_DMA_COHERENCE_H
+#define __ASM_MACH_BCM63XX_DMA_COHERENCE_H
+
+#include <asm/bmips.h>
+
+#define plat_post_dma_flush	bmips_post_dma_flush
+
+#include <asm/mach-generic/dma-coherence.h>
+
+#endif /* __ASM_MACH_BCM63XX_DMA_COHERENCE_H */