realtek: d-link: add support for dgs-1210-28mp-f

realtek: d-link: add support for dgs-1210-28mp-f

General hardware info:
----------------------

D-Link DGS-1210-28MP rev. F1 is a switch with 24 ethernet ports and 4
combo ports, all ports Gbit capable. It is based on a RTL8382 SoC @ 500MHz,
DRAM 128MB and 32MB flash. 24 ethernet ports are 802.3af/at PoE capable
with a total PoE power budget of 370W.

Power over Ethernet:
--------------------

The PSE hardware consists of three BCM59121 PSE chips, serving 8 ports
each. They are controlled by a Nuvoton MCU.
In order to enable PoE, the realtek-poe package is required. It is
installed by default, but currently it requires the manual editing of
/etc/config/poe. Keep in mind that the port number assignment does not
match on this switch, alway 8 ports are in reversed order: 8-1, 16-9 and
24-17.

LEDs and Buttons:
-----------------

On stock firmware, the mode button is supposed to switch the LED indicators
of all port LEDs between Link Activity and PoE status. The currently
selected mode is visualized using the respective LEDs. PoE Max indicates
that the maximum PoE budget has been reached.
Since there is currently no support for this behavior, these LEDs and
the mode button can be used independently.

Serial connection:
------------------
The UART for the SoC (115200 8N1) is available via unpopulated standard
0.1" pin header marked J6. Pin1 is marked with arrow and square.

Pin 1: Vcc 3.3V
Pin 2: Tx
Pin 3: Rx
Pin 4: Gnd

OEM installation from Web Interface:
------------------------------------

  1. Make sure you are booting using OEM in image 2 slot. If not, switch to
     image2 using the menus
        System > Firmware Information > Boot from image2
        Tools > reboot
  2. Upload image in vendor firmware via Tools > Backup / Upgrade
     Firmware > image1
  3. Toogle startup image via System > Firmware Information > Boot from
     image1
  4. Tools > reboot

Other installation methods not tested, but since the device shares the
board with the DGS-1210-28, the following should work:

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

  1. Press Escape key during `Hit Esc key to stop autoboot` prompt
  2. Press CTRL+C keys to get into real U-Boot prompt
  3. Init network with `rtk network on` command
  4. Load image with `tftpboot 0x8f000000
     openwrt-rtl838x-generic-d-link_dgs-1210-28mp-f-initramfs-kernel.bin`
     command
  5. Boot the image with `bootm` command

Signed-off-by: Andreas Böhler <dev@aboehler.at>