ramips: add support for Linksys EA7300 v1

ramips: add support for Linksys EA7300 v1

Specifications:

* SoC:      MediaTek MT7621A              (880 MHz 2c/4t)
* RAM:      Nanya NT5CC128M16IP-DIT       (256M DDR3-1600)
* Flash:    Macronix MX30LF1G18AC-TI      (128M NAND)
* Eth:      MediaTek MT7621A              (10/100/1000 Mbps x5)
* Radio:    MT7615N                       (2.4 GHz & 5 GHz)
            4 antennae: 1 internal and 3 non-deatachable
* USB:      3.0 (x1)
* LEDs:
    White   (x1 logo)
    Green   (x6 eth + wps)
    Orange  (x5, hardware-bound)
* Buttons:
    Reset   (x1)
    WPS     (x1)

Everything works! Been running it for a couple weeks now and haven't had
any problems. Please let me know if you run into any.

Installation:

Flash factory image through GUI.

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.

Signed-off-by: Santiago Rodriguez-Papa <contact@rodsan.dev>
[use v1 only, minor DTS adjustments, use LINKSYS_HWNAME and add it to
DEVICE_VARS, wrap DEVICE_PACKAGES, adjust commit message/title]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>