ipq40xx: add support for EnGenius EMR3500

ipq40xx: add support for EnGenius EMR3500

SOC:     IPQ4018 / QCA Dakota
CPU:     Quad-Core ARMv7 Processor rev 5 (v7l) Cortex-A7
DRAM:    256 MiB
NOR:     32 MiB
ETH:     Qualcomm Atheros QCA8072 (2 ports)
USB:     1 x 2.0 (Host controller in the SoC)
WLAN1:   Qualcomm Atheros QCA4018 2.4GHz 802.11bgn 2:2x2
WLAN2:   Qualcomm Atheros QCA4018 5GHz 802.11a/n/ac 2:2x2
INPUT:   RESET Button
LEDS:    White, Blue, Red, Orange

Flash instruction:

From EnGenius firmware to OpenWrt firmware:

In Firmware Upgrade page, upgrade your openwrt-ipq40xx-generic-engenius_emr3500-squashfs-factory.bin directly.

From OpenWrt firmware to EnGenius firmware:

1. Setup a TFTP server on your computer and configure static IP to 192.168.99.8
   Put the EnGenius firmware in the TFTP server directory on your computer.
2. Power up EMR3500. Press 4 and then press any key to enter u-boot.
3. Download EnGenius firmware
   (IPQ40xx) # tftpboot 0x84000000 openwrt-ipq40xx-emr3500-nor-fw-s.img
4. Flash the firmware
   (IPQ40xx) # imgaddr=0x84000000 && source 0x84000000:script
5. Reboot
   (IPQ40xx) # reset

Signed-off-by: Yen-Ting-Shen <frank.shen@senao.com>
[squashed update patch, updated to 5.4, dropped BOARD_NAME,
migrated to SOC]
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>