ramips: add support for Ruijie RG-EW1300G (V1.00)

ramips: add support for Ruijie RG-EW1300G (V1.00)

The RG-EW1300G is a router with 1 x WAN and 3 x LAN gigabit ports.
The router runs on Ruijie OS by default.

- Specifications:

* SoC: MT7621A
* RAM: 128MB DDR3
* Flash: 16MB SPI NOR flash (GD25Q128C)
* WiFi0: Mediatek MT7615 2.4GHz 802.11b/g/n
* WiFi1: Mediatek MT7615 5GHz 802.11ac
* Ethernet: MT7530, 4x 1000Base-T.
* UART: Serial console - As marked on PCB, baudrate is 57600. DO NOT CONNECT 3.3V.
* Buttons: Reset, WPS.
* LED: Programmable LEDs via GPIO working for Red+Green status, and Mesh/WPS at the rear of the chassis.

- Default Flash:
```
GD25Q128C(c8 40180000) (16384 Kbytes)
mtd .name = raspi, .size = 0x01000000 (16M) .erasesize = 0x00010000 (64K) .numeraseregions = 0
6 cmdlinepart partitions found on MTD device raspi
Creating 6 MTD partitions on "raspi":
0x000000000000-0x000000050000 : "u-boot"
0x000000050000-0x000000060000 : "u-boot-env"
0x000000060000-0x000000070000 : "Factory"
0x000000070000-0x000000080000 : "product_info"
0x000000080000-0x000000090000 : "kdump"
0x000000090000-0x000001000000 : "firmware"
0x00000031a847-0x000001000000 : "rootfs"
mtd: partition "rootfs" must either start or end on erase block boundary or be smaller than an erase block -- forcing read-only
mtd: partition "rootfs_data" created automatically, ofs=0xae0000, len=0x520000
0x000000ae0000-0x000001000000 : "rootfs_data"
register mt_drv
```

```
cat /proc/mtd
dev:    size   erasesize  name
mtd0: 00050000 00010000 "u-boot"
mtd1: 00010000 00010000 "u-boot-env"
mtd2: 00010000 00010000 "factory"
mtd3: 00010000 00010000 "product_info"
mtd4: 00010000 00010000 "kdump"
mtd5: 00f70000 00010000 "firmware"
mtd6: 0028a847 00010000 "kernel"
mtd7: 00ce57b9 00010000 "rootfs"
mtd8: 00520000 00010000 "rootfs_data"
```

- Installation:

1. Open the case, solder to the marked 4 pin header
2. Connect it to a USB-UART TTL (do not connect to 3.3v)
3. Open a terminal with baud 57600.
4. Power on device, and repeatedly press "2" key to catch bootloader option
5. Set IP, TFTP server IP, and image file to load (eg, openwrt-ramips-mt7621-ruijie_rg-ew1300g-v1-squashfs-sysupgrade.bin)
6. System will reboot into OpenWRT.

Signed-off-by: Matt Brent <git@mattzfiber.co.za>
Link: https://github.com/openwrt/openwrt/pull/21864
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>