ipq806x: add support for Buffalo WXR-2533DHP

Buffalo WXR-2533DHP is a 2.4/5 GHz band 11ac router, based on Qualcomm
IPQ8064.

The U-Boot on WXR-2533DHP employs a complicated dual firmware
protection scheme against corruptions of the kernel and rootfs
images. See the notes in buffalo.sh for details.

specifications:

- Qualcomm IPQ8064 (384 - 1,400 MHz, 2C2T)
- 512 MB of RAM (DDR3)
- 256 MB of Flash (NAND)
- 4T4R 2.4/5 GHz Wlan (QCA9980)
- 5x 10/100/1000 Mbps Ethernet
- 10x LEDs, 8x keys (6x buttons, 2x slide-switches)
- 2x USB 3.0 Type-A
- 12VDC/4A AC Adapter
- UART through-hole on PCB
  - J3: Vcc, GND, TX, RX from USB port side
  - 115200n8

Boot instructions for the initramfs image:

1. Prepare the TFTP server with the initramfs image renamed to
   "wxr2300dhp-initramfs.uImage" and IP address "192.168.11.10".
2. Press the "AOSS" button while powering on the WXR-2533DHP.
3. Wait until the "Wireless" LED flashes before releasing the AOSS button.
   The WXR-2533DHP will grab the image from TFTP server and will boot it.

Flashing instructions:

To persistently write the firmware, flash an openwrt sysupgrade image
from inside the initramfs, for example transfer
via `scp <sysupgrade> root@192.168.1.1:/tmp` and flash on the device
with `sysupgrade -n /tmp/<sysupgrade>`. Then wait ~120 seconds to
let it finish the flashing process.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com> [reworded message]