realtek: add support for Netgear GS108T v3

realtek: add support for Netgear GS108T v3

The Netgear GS108T v3 is an 8 port gigabit switch with PoE-PD support
on port 1.  The two prior versions were built using eCos and are not
currently compatible with OpenWRT.

The GS108T v3 is quite similar to both the GS110TPP v1 and GS110TP v3,
all of which use the same firmware image from Netgear.  For this reason,
the device tree is identical aside from the model and compatible values.

All of the above feature a dual firmware layout, referred to as Image0
and Image1 in the Netgear firmware.

Hardware specification
----------------------

 * RTL8380M SoC, 1 MIPS 4KEc core @ 500MHz
 * 128MB DDR3-1600 DRAM (Winbond W631GG8MB-12)
 * 32MB 3v NOR SPI Flash (Macronix MX25L25635F or Winbond W25Q256JVFIQ)
 * RTL8231 GPIO extender to control the LEDs and the reset button
 * 8 x 10/100/1000BASE-T ports, internal PHY (RTL8218B)
 * UART (115200 8N1) via unpopulated standard 0.1" pin header marked J1
 * Power is supplied via a 12V 1A barrel connector or 802.3af

UART pinout
-----------

J1 | [o]ooo
      ^ ||`------ GND
      | |`------- RX         [TX out of the serial adapter]
      | `-------- TX         [RX into the serial adapter]
      `---------- Vcc (3V3)  [the square pin]

The through holes are filled with PB-free solder which melts at 375C.
They can also be drilled using a 0.9mm bit.

Build configuration
-------------------

 * Target System: Realtek MIPS
 * Target Profile: Netgear GS108T v3
 * Target Images -> ramdisk -> Compression: lzma
 * Disable other target images

Boot initramfs image from U-Boot
--------------------------------

 1. Press the Escape key at the `Hit Esc key to stop autoboot` prompt
 2. Init network with `rtk network on` command
 3. Load image with `tftpboot 0x8f000000 openwrt-realtek-generic-netgear_gs108t-v3-initramfs-kernel.bin` command
 4. Boot the image with `bootm` command

The switch defaults to IP 192.168.1.1 and tries to fetch the image via
TFTP from 192.168.1.111.

Updating the installed firmware
-------------------------------

The OpenWRT ramdisk image can be flashed directly from the Netgear UI.
The Image0 slot should be used in order to enable sysupgrade.

As with similar switches, changing the active boot partition can be
accomplished in U-Boot as follows:

 1. Press the Escape key at the `Hit Esc key to stop autoboot` prompt
 2. Run `setsys bootpartition {0|1}` to select the boot partition
 3. Run `savesys` followed by `boota` to proceed with the boot process

Signed-off-by: Michael Mohr <akihana@gmail.com>