ramips: add support for Youku YK-L2

ramips: add support for Youku YK-L2

Hardware spec:

CPU: MTK MT7621A
RAM: 256MB
ROM: 16MB SPI Flash
WiFi: MT7603EN + MT7612EN
Button: 2 buttons (reset, wps)
LED: 8 LEDs (Power 2G 5G WPS Internet LAN1 LAN2 USB)
Ethernet: 3 ports, 2 LAN + 1 WAN
Other: USB3.0

Flashing instructions:

Visit the openwrt forum topic for this router:
https://forum.openwrt.org/t/add-openwrt-support-for-youku-yk-l2/34692

to get the bootloader and unlock firmware.

0. upgrade your router with the telnet firmware via the
   firmware upgrade page on the webui.

1. telnet 192.168.11.1 from your PC

2. Download the pb-boot-youku_l2-20190317-61b6d33.bin and transfer
   it to the /tmp directory of the router.

3. mtd write /tmp/pb-boot-youku_l2-20190317-61b6d33.bin Bootloader

4. turn off the power

5. Push the reset button while turning on the router and
   wait until LED start blinking (~10sec.)

6. Connect Ethernet port and goto http://192.168.1.1.

7. Upload the firmware to firmware restore page in webui.

Signed-off-by: Zhao Yu <574249312@qq.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com> [rewrote the
flashing instructions, fixed author]