rtl838x: add support for D-Link DGS-1210-28

rtl838x: add support for D-Link DGS-1210-28

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

 * RTL8382M SoC, 1 MIPS 4KEc core @ 500MHz
 * 128MB DRAM
 * 32MB NOR Flash (MX25L25635E)
 * 24 x 10/100/1000BASE-T ports
    - Internal PHY with 8 ports (RTL8218B)
    - Two external PHYs with 8 ports each (RTL8218B)
 * 4 x Gigabit RJ45/SFP Combo ports
    - External PHY with 4 SFP ports (RTL8214FC)
 * Power LED
 * Reset button on front panel
 * UART (115200 8N1) via unpopulated standard 0.1" pin header marked J6

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

  [oooo]J3 [o]ooo|J6
    |       ^ ||`------ GND
    |       | |`------- RX
    |       | `-------- TX
    |       `---------- Vcc (3V3)
    |
    `------------------ J3 is power input connector nearby J6 UART

 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-1210-28-initramfs-kernel.bin` command
  5. Boot the image with `bootm` command

To install, upload the sysupgrade image to the OEM webpage or sysupgrade
from the system running from initramfs image.

It has been developed and tested on device with F1 revision.

Signed-off-by: Petr Štetiar <ynezz@true.cz>