ramips: add support for TRENDnet TEW-638APB V2

This patch add support for the TRENDnet TEW-638APB V2.

Specification:
- SoC: Ralink SoC RT3052F
- Flash: 4MB
- RAM: 32MB
- Ethernet: 1x LAN (100 Mbps)
- Wireless: 2.4GHz b/g/n, 2x external antenna
- Buttons: 1x Reset, 1x WPS
- LEDs: Power (green), Ethernet (green), WPS (green and orange),
  Wireless (green)
- UART: 1x UART on PCB (3.3V, GND, RX, TX) - 57600 8N1

Installation

via vendor firmware:
- upload sysupgrade.bin image

via TFTP:
- stop uboot into tftp-load into option "2"
- upload sysupgrade.bin image

Signed-off-by: Pavlo Samko <bulldozerbsg@gmail.com>