[][kernel][gphy][Add CH395 solution to dsa, gsw]

Add solution of CH395, which is an ethernet IC chip
from WCH company, to dsa, gsw for mt7981, mt7986.
1. For dsa, CR will be written in mediatek_ge.c
2. For gsw, CR will be written in mt7531.c
3. Enable mt7986 mediatek_ge.c config

This patch will adjust RX min/max gain. Without this patch,
it may cause a link up fail for CH395 chip under 100Mbps.
In order to enhance the stability of your products,
it's better to merge this patch in your codebase.

Change-Id: If07b4ffc060defb5460ecf98339c1cdd3520a128
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7858068
3 files changed