ramips: add support for ASUS RP-AC87

ramips: add support for ASUS RP-AC87

Asus RP-AC87 ac2600 Repeater
2.4GHz 800Mbps
5GHz 1733Mbps

Hardware specifications:
SoC: MT7621A 2 cores 4 threads @880MHz
WiFi2G: MT7615E 2G 4x4 b/g/n
Wifi5G: MT7615E 5G 4x4 n/ac
DRAM: 128MB DDR3 @1200mhz
Flash: 16MB MX25L12805D SPI-NOR
LAN/WAN: MT7530 1x1000M

MAC addresses as verified by OEM firmware:
use address source
Lan/W5G *:B0 factory 0x8004 (label)
W2G *:B4 factory 0x0

Installation:

Asus windows recovery tool:

install the Asus firmware restoration utility
unplug the router, hold the reset button while powering it on
release when the power LED flashes slowly
specify a static IP on your computer:
IP address: 192.168.1.75
Subnet mask 255.255.255.0
Start the Asus firmware restoration utility, specify the factory image
and press upload
Do not power off the device after OpenWrt has booted until the LED flashing.
TFTP Recovery method:

set computer to a static ip, 192.168.1.2
connect computer to the LAN 1 port of the router
hold the reset button while powering on the router for a few seconds
send firmware image using a tftp client; i.e from linux:
$ tftp
tftp> binary
tftp> connect 192.168.1.1
tftp> put factory.bin
tftp> quit

Signed-off-by: Tamas Balogh <tamasbalogh@hotmail.com>