ramips: add support for ASUS RT-AC85U

ramips: add support for ASUS RT-AC85U

This patch adds support for the ASUS RT-AC85U wireless router.
It is a dual-band gigabit router based on the MediaTek MT7621AT SoC.

MAC Address Layout:
- LAN/WAN/2.4G/5G MAC addresses are derived from the
  factory partition at offset 0xe000.

Hardware Specifications:
- SoC: MediaTek MT7621AT (880 MHz, 2C/4T)
- RAM: 128 MB DDR3
- Flash: 128 MB Winbond W29N01HV NAND
- WI1: MediaTek MT7615E (2.4 GHz, 4x4)
- WI2: MediaTek MT7615E (5 GHz, 4x4)
- Switch: MediaTek MT7530 (Embedded Gigabit Switch)
- Ports: 1x WAN, 4x LAN (10/100/1000 Mbps), 1x USB 3.0
- LEDs: Power, LAN, WAN, 2.4G, 5G, USB, WPS
- Buttons: WPS, Reset

Installation / Flashing guide:
1. Power off the device.
2. Hold Reset button and power on to enter ASUS Restoration mode (slow flash LED).
3. Set your computer's static IP to 192.168.1.x (e.g., 192.168.1.10).
4. Upload the OpenWrt initramfs image via ASUS Restoration tool to 192.168.1.1.
5. Wait for the device to boot into the temporary OpenWrt environment.
6. Access LuCI (192.168.1.1) and flash sysupgrade image to make it permanent.

Signed-off-by: YAJIMA Hideyuki <ursmtr@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23747
Signed-off-by: Robert Marko <robimarko@gmail.com>