ramips: add support for Linksys EA8100 v2

ramips: add support for Linksys EA8100 v2

Specifications:
- SoC: MT7621AT
- RAM: 256MB
- Flash: 128MB NAND
- Ethernet: 5 Gigabit ports
- WiFi: 2.4G/5G MT7615N
- USB: 1 USB 3.0, 1 USB 2.0

This device is very similar to the EA7300 v1/v2, EA7500 v2, and EA8100 v1.

Installation:

Upload the generated factory image through the factory web interface.

(following part taken from EA7300 v2 commit message:)

This might fail due to the A/B nature of this device. When flashing, OEM
firmware writes over the non-booted partition. If booted from 'A',
flashing over 'B' won't work. To get around this, you should flash the
OEM image over itself. This will then boot the router from 'B' and
allow you to flash OpenWRT without problems.

Reverting to factory firmware:

Hard-reset the router three times to force it to boot from 'B.' This is
where the stock firmware resides. To remove any traces of OpenWRT from
your router simply flash the OEM image at this point.

With thanks to Tom Wizetek (@wizetek) for testing.

Signed-off-by: Tee Hao Wei <angelsl@in04.sg>