ramips: add support for Mikrotik LtAP-2HnD

ramips: add support for Mikrotik LtAP-2HnD

Mikrotik LtAP-2HnD is a outdoor/automotive WLAN 4 router with integrated GPS
receiver and two mPCIe slots.

Specifications:
* SoC: MT7621A
* RAM: 128 MiB Nanya NT5CC64M16GP-DI
* Flash: 16 MiB winbond W25Q128JV
* WLAN:
  * Atheros AR9382 with power amplifier SKY 85330 (2x2 internal antennas,
    with RF switches for external connectors)
* Ethernet: 1 Gbps, single port
* USB Host: USB 2.0 Speeds
* Serial: 115200 baud
* LEDs: Power, System, GPS, 5* RSSI
*  mPCIe:
   * miniPCIe slot 1: PCIe and USB 2.0 Host (via switch shared with USB Host)
   * miniPCIe slot 2: USB 2.0 and 3.0
* SIM Cards:
  * Slot 1 Connected to mPCIe slot 1
  * Slot 2 and 3 connected to mPCIe slot 2 via switch
* GPS: MTK 3333 on serial port 2 (/dev/ttyS1), 115200 baud and PPS on gpio 14

gpios are exposed to /sys/class/gpio:

* usb-select: swithes USB 2.0 interface between external port and internal
mPCIe slot 1 default is the external USB interface
* gps-reset: resets the GPS interface chip
* sim-select: switches between sim slot 2 and 3 connected to mPCIe slot 2
* gps-ant-select: switches GPS antenna between internal antenna and SMA
connected antenna
* lte-reset: resets mPCIe slot 2

Flashing:
 TFTP boot initramfs image and then perform sysupgrade. Follow common
 MikroTik procedure as in https://openwrt.org/toh/mikrotik/common.

Signed-off-by: Arne Zachlod <arne@nerdkeller.org>