ath79: add support for ELECOM WAB-I1750-PS

ath79: add support for ELECOM WAB-I1750-PS

ELECOM WAB-I1750-PS is a 2.4/5 GHz band 11ac (Wi-Fi 5) access point,
based on QCA9558.

Specification:

- SoC              : Qualcomm Atheros QCA9558
- RAM              : DDR2 128 MiB (2x Winbond W9751G6KB251)
- Flash            : SPI-NOR 16 MiB (Macronix MX25L12835FMI-10G)
- WLAN             : 2.4/5 GHz 3T3R
  - 2.4 GHz        : Qualcomm Atheros QCA9558 (SoC)
  - 5 GHz          : Qualcomm Atheros QCA9880
- Ethernet         : 2x 10/100/1000 Mbps
  - phy ("PD")     : Atheros AR8035
  - phy ("PSE")    : Atheros AR8033
- LEDs/keys (GPIO) : 3x/3x
- UART             : 2x RJ-45 port
  - "SERVICE"      : TTL (3.3V)
    - port         : ttyS0
    - assignment   : 1:3.3V, 2:GND, 3:TX, 4:RX
    - settings     : 115200n8
    - note         : no compatibility with "Cisco console cable"
  - "SERIAL"       : RS232C (+-12V)
    - port         : ?
    - assignment   : 1:NC , 2:NC , 3:TXD, 4:GND,
                     5:GND, 6:RXD, 7:NC , 8:NC
    - settings     : 115200n8
    - note         : compatible with "Cisco console cable"
- Buzzer           : 1x GPIO-controlled
- USB              : 1x USB 2.0 Type-A
- Power            : DC jack or PoE
  - DC jack        : 12 VDC, 1.04 A (device only, rating)
  - PoE            : 802.3af/at, 48 VDC, 0.26 A (device only, rating)
  - note           : supports 802.3af supply on PSE (downstream) port
                     when powered by DC adapter or 802.3at PoE

Flash instruction using factory.bin image:

1. Boot WAB-I1750-PS without no upstream connection (or PoE connection
   without DHCP)
2. Access to the WebUI ("http://192.168.3.1") on the device and open
   firmware update page
   ("ツールボックス" -> "ファームウェア更新")
3. Select the OpenWrt factory.bin image and click update
   ("アップデート") button
4. Wait ~120 seconds to complete flashing

Revert to OEM firmware:

1. Download the latest OEM firmware
2. Remove 128 bytes(0x80) header from firmware image
3. Decode by xor with a pattern "8844a2d168b45a2d" (hex val)
4. Upload the decoded firmware to the device
5. Flash to "firmware" partition by mtd command
6. Reboot

Notes:

- To use the "SERVICE" port, the connection of 3.3V line is also
  required to enable console output.
  The uart line of "SERVICE" is branched out from the internal pin
  header with 74HC126D and 3.3V line is connected to OE pin on it.

- "SERIAL" port is provided by HS UART on QCA9558 SoC that has
  compatibility with qca,ar9330-uart, but QCA955x SoC's is not supported
  on Linux Kernel and OpenWrt.

- To supply 802.3af PoE on "PSE" port when powered by DC adapter, 12 VDC
  3.5 A adapter is recommended. (official: WAB-EX-ADP1)

MAC addresses:

Ethernet (PD, PSE): 00:90:FE:xx:xx:0A (Config, ethaddr (text))
2.4GHz            : 00:90:FE:xx:xx:0A (Config, ethaddr (text))
5GHz              : 00:90:FE:xx:xx:0B

[original work]
Signed-off-by: Yanase Yuki <dev@zpc.st>
[update for NVMEM and others]
Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>