ipq806x: add support for Netgear Nighthawk Pro Gaming XR500

ipq806x: add support for Netgear Nighthawk Pro Gaming XR500

This adds support for the Netgear Nighthawk Pro Gaming XR500.
It is the successor to the Netgear Nighthawk R7800 and shares almost
identical hardware to that device.
The stock firmware is a heavily modified version of OpenWRT.

Specifications:
  SoC: Qualcomm Atheros IPQ8065
  RAM: 512 MB
  Storage: 256 MiB NAND Flash
  Wireless: 2x Qualcomm Atheros QCA9984
  Ethernet: 2x 1000/100/10 dedicated interfaces
  Switch: 5x 1000/100/10 external ports
  USB: 2x 3.0 ports

More information:
Manufacturer page: https://www.netgear.com/gaming/xr500/
Almost identical to Netgear R7800
Differences (r7800 > xr500):
  Flash: 128MiB > 256MiB
  Removed esata
  swapped leds:
    usb1 (gpio 7 > 8)
    usb2 (gpio 8 > 26)
    guest/esata (gpio 26 > 7)

MAC addresses:

On the OEM firmware, the mac addresses are:

  WAN: *:50  art 0x6
  LAN: *:4f  art 0x0 (label)
  2G:  *:4f  art 0x0
  5G:  *:51  art 0xc

Installation:
Install via Web Interface (preferred):
Utilize openwrt-ipq806x-netgear_xr500-squashfs-factory.img

Install via TFTP recovery:
1.Turn off the power, push and hold the reset button (in a hole on
  backside) with a pin
2.Turn on the power and wait till power led starts flashing white
  (after it first flashes orange for a while)
3.Release the reset button and tftp the factory img in binary mode.
  The power led will stop flashing if you succeeded in transferring
  the image, and the router reboots rather quickly with the new
  firmware.
4.Try to ping the router (ping 192.168.1.1). If does not respond,
  then tftp will not work either.

Uploading the firmware image with a TFTP client
$ tftp 192.168.1.1
bin
put openwrt-ipq806x-netgear_xr500-squashfs-factory.img

Note:

The end of the last partition is at 0xee00000. This was chosen
by the initial author, but nobody was able to tell why this
particular arbitrary size was chosen. Since it's not leaving
too much empty space and it's the only issue left, let's just
keep it for now.

Based on work by Adam Hnat <adamhnat@gmail.com>
ref: https://github.com/openwrt/openwrt/pull/3215

Signed-off-by: Peter Geis <pgwipeout@gmail.com>
[squash commits, move common LEDs to DTSI, remove SPDX on old
 files, minor whitespace cleanup, commit message facelift,
 add MAC address overview, add Notes, fix MAC addresses,
 use generic name for partition nodes in DTS]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>