ramips: add support for Zbtlink ZBT-WG1602-V04

ramips: add support for Zbtlink ZBT-WG1602-V04

Description heavily based on commit
7e89421a7c3855d66a20350a5bf9ca4cc7a2caf9 by
Sergey Ryazanov <ryazanov.s.a@gmail.com> Details I cannot confirm have
been removed

Completed with great help from \x on IRC. Thanks, \x!

Zbtlink ZBT-WG1602-V04 is a Wi-Fi router intendend for use with WWAN
(UMTS/LTE/3G/4G) modems. The router board offers a couple of miniPCIe
slots with USB and SIM only and another one which is a pure miniPCIe
slot as well as five Gigabit Ethernet ports (4xLAN + WAN).

Specification:

* SoC: MT7621A
* RAM: 256/512 MiB
* Flash: 16/32 MiB
* Eth: 10/100/1000 Mbps Ethernet x5 ports (4xLAN + WAN)
* WLAN 2GHz: MT7603E (.11bgn, MIMO 2x2)
* WLAN 5GHz: MT7662E (.11nac, MIMO 2x2)
* WLAN Ants: detachable x2, shared by 2GHz & 5GHz radios
* miniPCIe: 2x slots with USB&SIM + 1x slot with regular PCIe bus
* WWAN Ants: detachable x4
* External storage: microSD (SDXC) slot
* USB: 3.0 Type-A port
* LED: 11 (5 per Eth phy, 3 SoC controlled, 2 WLAN 2/5 controlled,
  1 power indicator)
* Button: 1 (reset)
* UART: console (115200 baud)
* Power: DC jack (12 V / 2.5 A)

Additional HW information:

* SoC USB port 1 is shared by internal miniPCIe slot and external
  Type-A USB port, USB D+/D- lines are toggled between ports using a
  GPIO controlled DPDT switch.

Installation:

The kernel image can be installed directly onto the device via a browser
to 192.168.1.1 using the built in firmware recovery Web UI available.
It can be accessed by pushing the reset button in, applying power and
holding the reset button for approximately 10 seconds. When the kernel
image has been flashed, you can access LuCI and upload the sysupgrade
as normal.

Signed-off-by: Alexander Horner <ahorner@programmer.net>