ramips: add support for VoCore2

ramips: add support for VoCore2

This patch adds support for the VoCore VoCore2 and its complementary
"ultimate" dock.

Specifications:
- SoC: MediaTek MT7628AN (580MHz, ramips)
- RAM: 128MB DDR2 166MHz
- Storage: 16MB NOR SPI flash onboard + microSD slot on dock
- Wireless: Built into MT7628AN (mt76) with 1T1R firmware on VoCore2
            boards with onboard 1x chip antenna
- Ethernet: 1x100M (port0) on dock, 1x100M (port2) on PCB header
- Dock hardware:
  - USB 2.0 socket
  - MicroSD socket
  - 100Mbps Ethernet x1
  - 3.5mm headphone jack (TRRS) connected to Everest Semi ES8388 I2S
    DAC/ADC (support WIP)
  - Micro USB for power and console (UART2)

Initial installation:
- VoCore2 comes preinstalled with a fork of OpenWrt CC and AP on
  SSID "VoCore2"
- Connect to VoCore2 by Ethernet or Wi-Fi
- `ssh root@192.168.1.1` (password is "vocore")
- scp/wget/etc. LEDE sysupgrade.bin to VoCore2
- `sysupgrade -n <your image>.bin` (don't keep old config, as the
  original firmware uses Ralink SDK Wi-Fi drivers and not
  mt76+mac80211)
- after sysupgrade completes, Wi-Fi will be disabled by default so use
  Ethernet or the micro USB console to configure Wi-Fi again

Signed-off-by: Andrew Yong <me@ndoo.sg>