ipq40xx: Add support for Unielec U4019

ipq40xx: Add support for Unielec U4019

This commit adds support for the 32MB storage/512MB RAM version of the U4019
IPQ4019-based board from Unielec. The board has the following specifications:

* Qualcomm IPQ4019 (running at 717MHz)
* 512MB DDR3 RAM (optional 256MB/1GB)
* 32MB SPI NOR (optional 8/16MB or NAND)
* Five gigabit ports (Qualcomm QCA8075)
* 1x 2.4 GHz wifi (QCA4019 hw1.0)
* 1x 5 Ghz wifi (QCA4019 hw1.0)
* 1x mini-PCIe slot (only USB-pins connected)
* 1x SIM slot (mini-SIM)
* 1x USB2.0 port
* 1x button
* 1x controllable LED
* 1x micro SD-card reader

Working:
* Ethernet
* Wifi
* USB-port
* mini-PCIe slot + SIM slot
* Button
* Sysupgrade

Not working:
* SD card slot (no upstream support)

Installation instructions:

In order to install OpenWRT on the U4019, you need to go via the
initramfs-image. The installation steps are as follows:

* Connect to board via serial (header exposed and clearly marked).
* Interrupt bootloader by pressing a button.
* Copy the initramfs-image to your tftp folder, call the file C0A80079.img.
* Give the network interface connected to the U4019 the address
192.168.0.156/24.
* Start your tftp-server and run tftpboot on the board.
* Run bootm when the file has been transferred, to boot OpenWRT.
* Once OpenWRT has booted, copy the sysupgrade-image to the device and run
sysupgrade to install OpenWRT on the U4019.

Notes:

- Since IPQ4019 has been moved to 4.19, I have not added support for kernel
4.14.

- There is a bug with hardware encryption on IPQ4019, causing poor performance
with TCP and ipsec (see for example FS#2355). In order to improve performance,
I have disabled hardware encryption in the DTS. We can enable hw. enc. once/if
bug is fixed.

- In order for Ethernet to work, the phy has to be reset by setting gpio 47
low/high. Adding support for phy reset via gpio required patching the
mdio-driver, and the code added comes from the vendor driver. I do not know if
patching the driver is an acceptable approach or not.

v1->v2:
* Do not use wildcard as identifier in the board.d-scripts (thanks
Adrian Schmutzler).

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>