ramips: add support for Skylab SKW92A in EVB

ramips: add support for Skylab SKW92A in EVB

Specifically, SKW92A_E16, described here:

  http://www.skylabmodule.com/wp-content/uploads/SkyLab_SKW92A_V1.04_datasheet.pdf

Specification:
- MediaTek MT7628N/N (580 Mhz)
- 64 MB of RAM
- 16 MB of FLASH
- 2T2R 2.4 GHz
- 5x 10/100 Mbps Ethernet
- 2x u.FL
- Power by micro-USB connector at USB1 on EVB
- UART via micro-USB connector at USB3 on EVB (57600 8n1)
- 5x Ethernet LEDs
- 1x WLAN LEDs
- 1x WPS LED connected by jumper wire from I2S_CK on J20 to WPS_LED pin hole next
  to daughter board on EVB
- WPS/Reset button (S2 on EVB)
- RESET button (S1 on EVB) is *not* connected to RST hole next to daughter board

Flash instruction:

>From Skylab firmware:

1. Associate with SKYLAP_AP
2. In a browser, load: http://10.10.10.254/
3. Username/password: admin/admin
4. In web admin interface: Administration / Upload Firmware, browse to
   sysupgrade image, apply, flash will fail with a message:
   Not a valid firmware. *** Warning: "/var/tmpFW" has corrupted data!
5. Telnet to 10.10.10.254, drops you into a root shell with no credentials
6. # cd /var
7. # mtd_write -r write tmpFW mtd4
   Unlocking mtd4 ...
   Writing from tmpFW to mtd4 ... [e]
8. When flash has completed, you will have booted into your firmware.

>From U-boot via TFTP and initramfs:

1. Place openwrt-ramips-mt76x8-skw92a-initramfs-kernel.bin on a TFTP server
2. Connect to serial console at USB3 on EVB
3. Connect ethernet between port 1 (not WAN) and your TFTP server (e.g.
   192.168.11.20)
4. Start terminal software (e.g. screen /dev/ttyUSB0 57600) on PC
5. Apply power to EVB
6. Interrupt u-boot with keypress of "1"
7. At u-boot prompts:
   Input device IP (10.10.10.123) ==:192.168.11.21
   Input server IP (10.10.10.3) ==:192.168.11.20
   Input Linux Kernel filename (root_uImage) ==:openwrt-ramips-mt76x8-skw92a-initramfs-kernel.bin
8. Move ethernet to port 0 (WAN) on EVB
9. At new OpenWrt console shell, fetch squashfs-sysupgrade image and flash
   with sysupgrade.

>From U-boot via TFTP direct flash:

1. Place openwrt-ramips-mt76x8-skw92a-squashfs-sysupgrade.bin on a TFTP server
2. Connect to serial console at USB3 on EVB (57600 8N1)
3. Connect ethernet between port 1 (not WAN) an your TFTP server (e.g.
   192.168.11.20)
4. Start terminal software (e.g. screen /dev/ttyUSB0 57600) on PC
5. Apply power to EVB
6. Interrupt u-boot with keypress of "2"
7. At u-boot prompts:
   Warning!! Erase Linux in Flash then burn new one. Are you sure?(Y/N) Y
   Input device IP (10.10.10.123) ==:192.168.11.21
   Input server IP (10.10.10.3) ==:192.168.11.20
   Input Linux Kernel filename (root_uImage) ==:openwrt-ramips-mt76x8-skw92a-squashfs-sysupgrade.bin
8. When transfer is complete or as OpenWrt begins booting, move ethernet to
   port 0 (WAN).

Signed-off-by: Russell Senior <russell@personaltelco.net>