mediatek: filogic: add Acelink EW-7886CAX support

mediatek: filogic: add Acelink EW-7886CAX support

Acelink EW-7886CAX is an MT7986A (AKA Filogic 830) based access point.
It has 512 MiB of RAM, one 2.5 Gbps PoE (802.3at) Ethernet port and
on-SoC Wi-Fi. There is no printed MAC label (on my unit).

My unit came with Mediatek's firmware (based on OpenWrt 21.02)
installed. It was possible to simply upgrade using OpenWrt's sysupgrade
tool.

Another verified upgrade method is using U-Boot (requires UART). During
every boot there is "U-Boot Boot Menu". Selecting option "2. Upgrade
firmware" allows using U-Boot's tftp client to load and flash factory
image.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 07765f28b7911ca8207b6e7ec94508a5740c4697)