ipq40xx: add support for ZTE MF286D

ipq40xx: add support for ZTE MF286D

ZTE MF286D is a LTE router with four gigabit ethernet ports
and integrated QMI mPCIE modem.

Hardware specification:

  - CPU: IPQ4019
  - RAM: 256MB
  - Flash: NAND 128MB + NOR 2MB
  - WLAN1:  Qualcomm Atheros QCA4019 2.4GHz 802.11bgn 2x2:2
  - WLAN2:  Qualcomm Atheros QCA4019 5GHz 802.11anac 2x2:2
  - LTE: mPCIe cat 12 card (Modem chipset MDM9250)
  - LAN: 4 Gigabit Ports
  - USB: 1x USB2.0 (regular port). 1x USB3.0 (mpcie - used by the modem)
  - Serial console: X8 connector 115200 8n1

Known issues:

  - Many LEDs are driven by the modem. Only internal LEDs and wifi LEDs
    are driven by cpu.
  - Wifi LED is triggered by phy0tpt only
  - No VoIP support
  - LAN1/WAN port is configured as WAN
  - ZTE gives only one MAC per device. Use +1/+2/+3 increment for WAN
    and WLAN0/1

Opening the case:

1. Take of battery lid (no battery support for this model, battery cage
   is dummy).
2. Unscrew screw placed behind battery lid.
3. Take off back cover. It attached with multiple plastic clamps.
4. Unscrew four more screws hidden behind back case.
5. Remove front panel from blue chassis. There are more plastic
   clamps.
6. Unscrew two boards, which secures the PCB in the chassis.
7. Extract board from blue chassis.

Console connection (X8 connector):

1. Parameters: 115200 8N1
2. Pin description: (from closest pin to X8 descriptor to farthest)
    - VCC (3.3V)
    - TX
    - RX
    - GND

Install Instructions:

Serial + initramfs:
1. Place OpenWrt initramfs image for the device on a TFTP in
   the server's root. This example uses Server IP: 192.168.1.3
2. Connect serial console (115200,8n1) to X8 connector.
3. Connect TFTP server to RJ-45 port.
4. Stop in u-Boot and run u-Boot commands:
	setenv serverip 192.168.1.3
	setenv ipaddr 192.168.1.72
	set fdt_high 0x85000000
	tftp openwrt-ipq40xx-generic-zte_mf286d-initramfs-fit-zImage.itb
	bootm $loadaddr
5. Please make backup of original partitions, if you think about revert
   to stock.
6. Login via ssh or serial and remove stock partitions:
	ubiattach -m 9
	ubirmvol /dev/ubi0 -N ubi_rootfs
	ubirmvol /dev/ubi0 -N ubi_rootfs_data
7. Install image via "sysupgrade -n".

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
(cosmetic changes to the commit message)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>