realtek: add support for I-O DATA BSH-G24MB

realtek: add support for I-O DATA BSH-G24MB

I-O DATA BSH-G24MB is a 24 port gigabit switch, based on RTL8382M.

Specification:

- SoC		: Realtek RTL8382M
- RAM		: DDR2 128 MiB (Nanya NT5TU128M8HE-AC)
- Flash		: SPI-NOR 16 MiB (Macronix MX25L12835FM2I-10G)
- Ethernet	: 10/100/1000 Mbps x24
  - port 1-8	: RTL8218B
  - port 9-16	: RTL8218B (SoC)
  - port 17-24	: RTL8218B
- LEDs/Keys	: 2x, 1x
- UART		: pin header on PCB
  - JP2: 3.3V, TX, RX, GND from rear side
  - 115200n8
- Power		: 100 VAC, 50/60 Hz
  - Plug	: IEC 60320-C13

Flash instruction using sysupgrade image:

1. Boot BSH-G24MB normally
2. Connect BSH-G24MB to the DHCP enabled network
3. Find the device's IP address and open the WebUI and login
   Note: by default, the device obtains IP address from DHCP server of
         the network
4. Open firmware update page ("ファームウェア アップデート")
5. Rename the OpenWrt sysupgrade image to "bsh-g24mb_v100.image" and
   select it
6. Press apply ("適用") button to perform update
7. Wait ~150 seconds to complete flashing

Note:

- BSH-G24MB has a power-related LED ("電源"), but it's not connected to
  the GPIO of the SoC or RTL8231 and cannot be controlled. Instead of
  it, use system status LED on other than running-state.

- "sys_loop" LED indicates system status and loop-detection status in
  stock firmware.

- BSH-G24MB has 2x os-image partitions named as "RUNTIME"/"RUNTIME2" in
  16 MiB SPI-NOR flash and the size of image per partition is only
  6848 KiB. The secondary image is never used on stock firmware, so also
  use it on OpenWrt to get more space.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>