bcm53xx: add support for ASUS RT-AC3200 and ASUS RT-AC5300

bcm53xx: add support for ASUS RT-AC3200 and ASUS RT-AC5300

ASUS RT-AC3200 and ASUS RT-AC5300 are AC3200 and AC5300 routers,
respectively, featuring 5 Ethernet ports over the integrated Broadcom
switch.

ASUS RT-AC3200 hardware info:
* Processor: Broadcom BCM4709A0 dual-core @ 1.0 GHz
* Switch: BCM53012 in BCM4709A0
* DDR3 RAM: 256 MB
* Flash: 128 MB
* 2.4GHz: BCM43602 3x3 single chip 802.11b/g/n SoC
* 5GHz: BCM43602 3x3 two chips 802.11a/n/ac SoC
* Ports: 4 LAN Ports, 1 WAN Port

ASUS RT-AC5300 hardware info:
* Processor: Broadcom BCM4709C0 dual-core @ 1.4 GHz
* Switch: BCM53012 in BCM4709C0
* DDR3 RAM: 512 MB
* Flash: 128 MB
* 2.4GHz: BCM4366 4x4 single chip 802.11b/g/n SoC
* 5GHz: BCM4366 4x4 two chips 802.11a/n/ac SoC
* Ports: 4 LAN Ports, 1 WAN Port

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: Chester A. Unal <chester.a.unal@arinc9.com>