MIPS: Remove unnecessary platform dma helper functions

The semantics stay the same - on Cavium Octeon the functions were dead
code (it overrides the MIPS DMA ops) - on other platforms they contained
no code at all.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5720/


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