ipq40xx: add support for Linksys MR9000

ipq40xx: add support for Linksys MR9000

This pull request is based on
- the discussions in https://forum.openwrt.org/t/support-for-the-linksys-mr9000

Device Specs:
- Router is similar to EA8300 but with 512MB RAM and changed GPIOs
- IPQ4019
- Quad Core CPU
- 512 MB RAM
- 256 MB FLASH
- 4 LAN ports, 1 WAN port
- 1x2.4GHz (802.11n) and 2x5GHz (802.11c) wifi
- 3 LEDs (Red, blue, green) which are routed to one indicator at the top of the case
- 2 buttons (Reset, WPS)

Disassembling the device:
- There are 4 T10 screws at the bottom of the device which must be removed
- All 4 screws are located under the rubber feet

Serial interface:
- The serial interface is already populated on the device with a 5-pin header
- Pinout from front to back: 1:GND, 2: unknown, 3: RX, 4: TX, 5: VCC
- Settings: 115200, 8N1

MAC address layout:
- Same as on MR8300
  - MAC address is stored in the "devinfo" partition (entry "hw_mac_addr")
  - The MAC address on the label is identical with the one in the devinfo partition
  - The MAC address on the label is the WAN MAC address
  - LAN MAC address is the one from the label + 1
  - Wifi (2.4GHz) MAC is the one from the label + 2
  - Wifi (1st 5GHz) MAC is the one from the label + 3
  - Wifi (2nd 5GHz) MAC is the one from the label + 4

Migrating to OpenWrt requires multiple steps:
- Load and boot the initramfs image
- Adapt U-Boot settings to support bigger kernels
- Flash the sysupgrade image

Load and boot initramfs:
- Connect serial interface
- Set up a TFTP server on IP 192.168.1.254
- Copy openwrt-ipq40xx-generic-linksys_mr9000-initramfs-zImage.itb to TFTP server
- Rename file to C0A80101.img
- Boot up the device and stop in U-Boot
- Run the following U-Boot commands after a link has been established:
  tftp
  bootm
- Initramfs image is started now.

Adapt U-Boot settings to support bigger kernels:
- Run "fw_printenv" in the initramfs image  after booting
- There should be an entry kernsize=300000 which indicates the maximum size for the kernel is 3MB
- Execute "fw_setenv kernsize 500000" to increase the max kernel size to 5MB
- Check that the change are applied with "fw_printenv"

Flash the sysupgrade image:
- Default sysupgrade routine either with a initramfs image containing LuCI or via command line.

Revert back to OEM firmware:
- Flash the OEM firmware via sysupgrade
- Forced update is required

Signed-off-by: Karsten Rehn <rekados@posteo.de>
Signed-off-by: Roland Reinl <reinlroland+github@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22744
Signed-off-by: Robert Marko <robimarko@gmail.com>