ar71xx: add support for TP-Link RE355

ar71xx: add support for TP-Link RE355

This device is identical as TP-Link RE450
RE355 is a dual-band AC1200 router, based on Qualcomm/Atheros
QCA9558+QCA9880.

Specification:

720/600/200 MHz (CPU/DDR/AHB)
64/128 MB of RAM (DDR2)
8 MB of FLASH (SPI NOR)
3T3R 2.4 GHz
3T3R 5 GHz
1x 10/100/1000 Mbps Ethernet
7x LED, 3x button
UART header on PCB

Flash instruction:
Web:
Download lede-ar71xx-generic-archer-c60-v2-squashfs-factory.bin
and use OEM System Tools - Firmware Upgrade site.

Signed-off-by: Henryk Heisig <hyniu@o2.pl>