ramips: mt7621: add support for JDCloud RE-SP-01B

ramips: mt7621: add support for JDCloud RE-SP-01B

JDCloud RE-SP-01B is a dual-band WiFi 5 router based on the MT7621AT.

Specifications:
- SoC: MediaTek MT7621AT
- RAM: 512MB DDR3
- Flash: 32MB SPI NOR
- WiFi: MediaTek MT7603EN (2.4GHz), MediaTek MT7615N (5GHz)
- Ethernet: 1x WAN, 2x LAN (Gigabit Ethernet)
- LEDs: red, blue, green (GPIO controlled)
- Button: Reset (GPIO controlled)
- eMMC: Single onboard (32GB/64GB/128GB)
- USB: 1x USB 2.0 port

MAC Address Structure:
The MAC addresses share the structure DC:D8:7C:XX:XX:XX, where:
- WAN, LAN, and 2.4GHz WiFi: same as the label MAC address.
- 5GHz WiFi: label MAC address + 0x800000.

The manufacturer writes the label MAC address at different
offsets depending on the storage version of the device:

e.g.
128GB version:  &config + 0x442a
64GB version:   &config + 0x4429

So `get_mac_ascii()` is used here to search for the
base label MAC address of the device.

Ref:
https://github.com/openwrt/openwrt/pull/17409#discussion_r1899674262
https://github.com/immortalwrt/immortalwrt/commit/c0c480d

Flash Instruction:
A 3rd party bootloader is required to boot the image. You can
use a SOP16 test clip to burn the image/bootloader to the flash.

The official bootloader does provide a web recovery interface
which only accepts an official image. To access it, you will
need to hold the reset button and power on the device, set your
IP address to 192.168.68.2 and visit http://192.168.68.1.

Co-authored-by: Chukun Pan <amadeus@jmu.edu.cn>
Signed-off-by: Yijie Jin <jinyijie@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/17409
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>