[][kernel][common][eth][Add support for coherent DMA]

Add support for coherent DMA.

It improves performance by eliminating the need for a cache flush
on rx and tx.
In preparation for supporting WED (Wireless Ethernet Dispatch),
also add a function for disabling coherent DMA at runtime.

This modification refers to Linux upstream patch which came from
Felix Fietkau.
  - https://patchwork.kernel.org/project/linux-mediatek/patch/


Change-Id: I1dc9ec636ee0c780d8e8bd59673d36dd6bf1cbdb
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7214683
2 files changed