ramips: split Youku YK1 to YK-L1 and YK-L1c

ramips: split Youku YK1 to YK-L1 and YK-L1c

Device specifications:
* Model: Youku YK-L1/L1c
* CPU: MT7620A
* RAM: 128 MiB
* Flash: 32 MiB (YK-L1)/ 16 MiB (YK-L1c)
* LAN: 2* 10M/100M Ports
* WAN: 1* 10M/100M Port
* USB: 1* USB2.0
* SD: 1* MicroSD socket
* UART: 1* TTL, Baudrate 57600

Descriptions:
  Previous supported device YOUKU yk1 is actually Youku YK-L1. Though they look
  really different, the only hardware difference between the two models is flash
  size, YK-L1 has 32 MiB flash but YK-L1c has 16MiB. It seems that YK-L1c can
  compatible with YK-L1's firmware but it's better to split it to different models.

  It is easy to identify the models by looking at the label on the bottom of the
  device. The label has the model number "YK-L1" or "YK-L1c". Due to different flash
  sizes, YK-L1c that using previous YK-L1's firmware needs to apply "force update"
  to install compatible firmware, so please backup config file before system upgrade.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>
[use more specific name for DTSI]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>