ipq807x: add support for Compex WPQ873

ipq807x: add support for Compex WPQ873

The Compex WPQ873 is a development board with two M.2 B-key
 slots for cellular modems.

Device info:
 - IPQ8072A SoC
 - 512MiB RAM
 - 256MiB NAND flash
 - 8MiB SPI NOR
 - 3x 1GigE ports
 - 1x 2.5GigE port
 - 2.4GHz/5GHz AX WLAN
 - 1x USB 3.0 port
 - 1x M.2 B-key socket with PCIe 3.0
 - 1x M.2 B-key socket with PCIe 2.0 and USB 3.0
 - 4x SIM card slots
 - Bluetooth LE 5.0 (QCA4024)

Prerequisites
1) TFTP server
2) 3.3V USB to TTL cable for UART console
   2.54mm pitch 4-pin header for UART is readily provided on board, no modifications are necessary to access it
   TTL connector pinout: 2=TX, 3=RX, 4=GND
   Arrow marks pin 1 which is 3.3V
   Serial port settings: 115200 8N1 no flow control

The device will most likely ship with a QSDK-based firmware.

1. Power on device and interrupt u-boot to obtain u-boot CLI

2. set serverip to IP address of the TFTP server, for example:

        `setenv serverip 192.168.1.10`

3. Download image from TFTP server:

        `tftpboot 0x44000000 openwrt-ipq807x-generic-compex_wpq873-squashfs-factory.ubi`

4. Flash ubi image to both partitions and reset:

        `sf probe
         imxtract 0x44000000 ubi
         nand device 0
         nand erase 0x0 0x3400000
         nand erase 0x3c00000 0x3400000
         nand write $fileaddr 0x0 $filesize
         nand write $fileaddr 0x3c00000 $filesize
         reset`

Afterwards, you can use sysupgrade to flash new OpenWRT images.

Signed-off-by: Antti Nykänen <antti.nykanen@nokia.com>