ipq807x: Add ZyXEL NBG7815

ipq807x: Add ZyXEL NBG7815

ZyXEL NBG7815 is a premium 802.11ax "tri"-band router/AP.
Specifications:
* CPU: Qualcomm IPQ8072A Quad core Cortex-A53 2.2GHz
* RAM: 1 GB 2x Nanya NT5CC256M16ER-EK
* Storage:
        * 8MB serial flash Winbond W25Q64DW
        * 4GB eMMC flash Kingston EMMC04G-M627
* Ethernet:
        * 4x1G RJ45 ports (QCA8074A) with 1x status LED per port
        * 1x2.5G RJ45 port (QCA8081) with 1x status LED
        * 1x10G RJ45 port (AQR113C) with 1x status LED
* Switch: Qualcomm Atheros QCA8075
* WLAN:
        * 2.4GHz: Qualcomm QCN5024 4x4@40MHz 802.11b/g/n/ax 1147 Mbps PHY rate
        * 2x 5GHz: Qualcomm QCN5054 4x4 802.11a/b/g/n/ac/ax 2402 PHY rate
* Bluetooth CSR8811 using HSUART, currently unsupported
* USB: 1x USB3.0 Type-A port
* LED-s currently not supported:
        * White
        * Dark Blu
        * Amber
        * Purple
        * Purple and dark blue
        * Red
* Buttons:
        * 1x Soft reset
* Power: 12V DC Jack

Installation instructions:
* Disconnect WAN
* Reset device to factory defaults by pushing reset button 15 sec,
  LEDs should lit orange color.
* After 5-10 minutes, when the LEDs turn constant dark blue,
  put your LAN cable and connect at address 192.168.123.1 by telnet on port 23
* Login with
  NBG7815 login: root
  password: nbg7815@2019
* cd /tmp/ApplicationData
* wget -O openwrt-ipq807x-generic-zyxel_nbg7815-squashfs-sysupgrade.bin http://...
* wget https://github.com/itorK/nbg7815_tools/blob/main/flash_to_openwrt.sh
* run flash_to_openwrt.sh
If you can't use wget, you can transfer the files via nc.
See https://openwrt.org/inbox/toh/zyxel/nbg7815_armor_g5 for installation details.

Bluetooth usage:
* you need at least package bluez-utils, recommended bluez-daemon
* run following commands to enable and start
  hciattach  /dev/ttyMSM1 bcsp
  hciconfig hci0 up

Many thanks to itorK for his work on this device:
https://github.com/itorK/openwrt/tree/nbg7815

Reviewed-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: André Valentin <avalentin@marcant.net>