Revert "MIPS: BCM63xx: Provide a plat_post_dma_flush hook"

This reverts commit 3cf2954 ("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: Álvaro Fernández Rojas <[email protected]>
Reported-by: Jonas Gorski <[email protected]>
Signed-off-by: Florian Fainelli <[email protected]>
Cc: [email protected]
Cc: Kevin Cernekee <[email protected]>
Cc: Nicolas Schichan <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: Florian Fainelli <[email protected]>
Cc: [email protected]
Patchwork: https://patchwork.linux-mips.org/patch/10804/
Signed-off-by: Ralf Baechle <[email protected]>