bcm53xx: add support for Asus RT-AC88U

bcm53xx: add support for Asus RT-AC88U

Asus RT-AC88U is an AC3100 router featuring 9 Ethernet ports over the
integrated Broadcom and the external Realtek switch.

Hardware info:
* Processor: Broadcom BCM4709C0KFEBG dual-core @ 1.4 GHz
* Switch: BCM53012 in BCM4709C0KFEBG & external RTL8365MB
* DDR3 RAM: 512 MB
* Flash: 128 MB (ESMT F59L1G81LA-25T)
* 2.4GHz: BCM4366 4×4 2.4/5G single chip 802.11ac SoC
* 5GHz: BCM4366 4×4 2.4/5G single chip 802.11ac SoC
* Ports: 8 Ports, 1 WAN Ports

Flashing instructions:
* Boot to CFE Recovery Mode by holding the reset button while power-on.
* Connect to the router with an ethernet cable.
* Set IPv4 address of the computer to 192.168.1.2 subnet 255.255.255.0.
* Head to http://192.168.1.1.
* Reset NVRAM.
* Upload the OpenWrt image.

CFE bootloader may reject flashing the image due to image integrity check.
In that case, follow the instructions below.

* Rename the OpenWrt image as firmware.trx.
* Run a TFTP server and make it serve the firmware.trx file.
* Run the URL below on a browser or curl.
  http://192.168.1.1/do.htm?cmd=flash+-noheader+192.168.1.2:firmware.trx+flash0.trx

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
[rmilecki: mark BROKEN until we sort out nvram & CFE recovery]
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>