Revert "MIPS: BCM63xx: Provide a plat_post_dma_flush hook"

This reverts commit 3cf29543 ("MIPS:
BCM63xx: Provide a plat_post_dma_flush hook") since this commit was
found to prevent BCM6358 (early BMIPS4350 cores) and some BCM6368
(BMIPS4380 cores) from booting reliably.

Alvaro was able to track this down to an issue specifically located to
devices that use the second thread (TP1) when booting. Since BCM63xx did
not have a need for plat_post_dma_flush() hook before, let's just keep
things the way they were.
Reported-by: NÁlvaro Fernández Rojas <noltari@gmail.com>
Reported-by: NJonas Gorski <jogo@openwrt.org>
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>
Cc: stable@vger.kernel.org
Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: Nicolas Schichan <nschichan@freebox.fr>
Cc: linux-mips@linux-mips.org
Cc: blogic@openwrt.org
Cc: noltari@gmail.com
Cc: jogo@openwrt.org
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: stable@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/10804/Signed-off-by: NRalf 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 */