qualcommax: ipq807x: add Netgear RBx750 support

qualcommax: ipq807x: add Netgear RBx750 support

Netgear RBx750 are tri band, 2.4GHz and 2x 5GHz, 8 stream 802.11ax mesh
devices from the Orbi series. The RBR750 is a router with a WAN and 3 LAN
ports. The RBS750 is a satellite without WAN port, only 2 LAN ports and
half the flash. The hardware is otherwise identical. They were sold in
kits as RBK752-RBK757, with one router and 1-6 satellites.

Hardware:
* SoC: Qualcomm IPQ8074
* RAM: 1GiB 1x Samsung
* Flash: 512MiB Winbond W29N04GZ or 256MiB Winbond W29N02GZ
* WLAN 2.4GHz: QCN5024 2x2:2 b/g/n/ax
* WLAN 5GHz Low Band: QCN5054 2x2:2 a/n/ac/ax 5180-5320MHz
* WLAN 5GHz High Band: QCN5054 4x4:4 a/n/ac/ax 5500-5700MHz
* Ethernet: QCA8075 switch with 1 WAN and 3 LAN ports or 2 LAN ports
* Serial Config: 3.3V TTL 115200-8-N-1, internal populated header
* Serial Layout: Bottom <- RX, TX, GND, 3.3V (don't connect) -> Top
* LEDs: green/red power, white/red/green/blue status
* Buttons: 1x Reset, 1x WPS

MAC addresses:
LAN1: base address on label
LAN2: base + 1
LAN3: base + 2
WAN: base + 1
2.4GHz: base + 2
5GHz-Low: base + 3
5GHz-High: base + 4

Flashing Notes:
The stock firmware images are signed. Both the bootloader and the stock
web interface check the signature and will fail to boot/flash.
The bootloader automatically does NMRP when a gigabit LAN connection is
present. The stock and factory images contain a U-Boot script that is
executed when flashing using NMRP. This is used to alter and persist the
U-Boot env with a boot command that works with unsigned firmware.

Install OpenWrt:
* Get the nmrpflash utility [0] and OpenWrt factory image
* Find network interface to use: nmrpflash -L
* Start nmrpflash: nmrpflash -i interface -f openwrt-...-factory.img
* Connect the device LAN port closest to the power jack to the same
  network using gigabit
* Plug the device in and wait for the bootloader to flash
* Unplug and replug the device once the power LED blinks amber

Revert to Stock:
The boot command needs to be reverted before flashing the stock firmware,
otherwise it will fail to boot and get stuck in recovery mode (red power
LED flashing).

* Run: fw_setenv bootcmd bootipq
* Restart the device
* Flash the stock firmware RBx750-Va.b.c.d.img using nmrpflash

[0]: https://github.com/jclehner/nmrpflash

Signed-off-by: Michael Lotz <mmlr@mlotz.ch>
Link: https://github.com/openwrt/openwrt/pull/21938
Signed-off-by: Robert Marko <robimarko@gmail.com>