ipq806x: add support for GL.iNet GL-B1300

ipq806x: add support for GL.iNet GL-B1300

This patch adds support for GL.iNet GL-B1300

Specification:
- SOC:        IPQ4028 / QCA Dakota
- RAM:        256 MiB
- FLASH:      32 MiB
- ETH:        Qualcomm Atheros QCA8075 Gigabit Switch (2 x LAN, 1 x WAN)
- USB:        1 x 3.0 (via Synopsys DesignWare DWC3 controller in the SoC)
- WLAN1:      Qualcomm Atheros QCA4028 2.4GHz 802.11bgn 2:2x2
- WLAN2:      Qualcomm Atheros QCA4028 5GHz 802.11a/n/ac 2:2x2
- INPUT:      one reset and one WPS button
- LEDS:       3 leds: Power, WIFI(only for 2.4G currently), and one reserved
- UART:       1 x UART on PCB (3.3V, TX, RX, GND) - 115200 8N1

Installation:
Method 1:
- use serial port to stop uboot
- uboot command: run lf
Method 2:
- push down reset button and power on
- wait until three leds constantly on then release
- upgrade by uboot web at http://192.168.1.1
Note:
- the sysupgrade image need to be renamed to lede-gl-b1300.bin in both method.
- the sysupgrade image can be automatically downloaded if tftp server at
  192.168.1.2 have that file.
- the wifi led will be flashing when writing image.

Signed-off-by: Dongming Han <handongming@gl-inet.com>