ramips: add support for Zbtlink ZBT-WG1602

ramips: add support for Zbtlink ZBT-WG1602

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

Specification:

* SoC: MT7621A
* RAM: 256/512 MiB
* Flash: 16/32 MiB (SPI NOR)
* external watchdog (looks like Torexsemi XC6131B)
* Eth: 10/100/1000 Mbps Ethernet x5 ports (4xLAN + WAN)
* WLAN 2GHz: MT7603EN (.11n, MIMO 2x2)
* WLAN 5GHz: MT7612EN (.11ac, 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: 2.0 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.
* Power of the USB enabled miniPCIe slots can be individually controlled
  using dedicated GPIO lines.
* Vendor firmware feeds the external watchdog with 1s pulses. GPIO
  watchdog driver is able to either generate a 1us pulses or toggle the
  output line. 1us is not enough for the external watchod timer, so
  the line toggling driver mode is utilized.

Installation:

Vendor's firmware is OpenWrt (LEDE) based, so the sysupgrade image can
be directly used to install OpenWrt. Firmware must be upgraded using the
'force' and 'do not save configuration' command line options (or
correspondig web interface checkboxes) since the vendor firmware is from
the pre-DSA era.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>