ramips: add support for ASUS RT-N65U

ramips: add support for ASUS RT-N65U

ASUS RT-N65U is a 2.4/5 GHz band 11n (Wi-Fi 4) router, based on RT3883.

Specification:
- CPU1: Ralink RT3883 @ 500MHz, WiSoC 5GHz
- CPU2: Ralink RT3352 @ 400MHz, WiSoC 2.4GHz
- RAM1: 128MB
- RAM2: 16MB
- Flash: Macronix MX25L12805D (16MB)
- WLAN: Ralink RT3883 (5GHz 3T3R), Ralink RT3352 (2.4GHz 2T2R)
- Switch: Realtek RTL8367RB
- 4x 10/100/1000 Mbps Ethernet
- 1x 10/100/1000 Mbps WAN
- USB: ASMedia ASM1042 USB 3.0 Controller
- PMU: UTC UZ1086L-ADJ
- 6x LED, 2x button

Installation (TFTP):
1. Power off the router.
2. Press and hold the Reset button.
3. Power on the router.
4. Wait 5 seconds, then release the Reset button.
5. Connect your PC to the router with Ethernet cable.
6. Configure a static IP address in the `192.168.1.0/24` subnet (except `192.168.1.1`).
7. Configure your TFTP client to use binary (octet) transfer mode.
8. Upload the firmware image (OpenWrt, stock, or Padavan) to `192.168.1.1`.
   The TFTP server accepts any filename.
9. Wait for the firmware flashing process to complete.

WARNING: Flashing the stock firmware after using Padavan may brick
your router due to configuration partition incompatibility.

MAC address layout:
- Wi-Fi 2.4GHz & WAN: label MAC
- Wi-Fi 5GHz & LAN: label MAC + 4

MTD layout:
```
mtd0: 00030000 00010000 "u-boot"
mtd1: 00010000 00010000 "config"
mtd2: 00010000 00010000 "factory"
mtd3: 00fb0000 00010000 "firmware"
mtd4: 001ee3b3 00010000 "kernel"
mtd5: 00dc1c4d 00010000 "rootfs"
mtd6: 00960000 00010000 "rootfs_data"
```

Known issues:
- The RT3352 2.4 GHz iNIC is unsupported because it requires a
  proprietary firmware blob and kernel support
- The Wi-Fi LEDs cannot be disabled independently because they share
  the LED power rail controlled by GPIO10

This work is based on the earlier draft support by Andrej Valek.

Tested by 4PDA users

Signed-off-by: Andrej Valek <andy@skyrain.eu>
Signed-off-by: Andrey Trishin <zatrit@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/24343
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>