realtek: add support for HPE 1920 series

realtek: add support for HPE 1920 series

Hardware information:
---------------------

- HPE 1920-8G:
  - RTL8380 SoC
  - 8 Gigabit RJ45 ports (built-in RTL8218B)
  - 2 SFP ports (built-in SerDes)

- HPE 1920-16G / HPE 1920-24G (same board):
  - RTL8382 SoC
  - 16/24 Gigabit RJ45 ports (built-in RTL8218B, 1/2 external RTL8218D)
  - 4 SFP ports (external RTL8214FC)

- Common:
  - RJ45 RS232 port on front panel
  - 32 MiB NOR Flash
  - 128 MiB DDR3 DRAM
  - PT7A7514 watchdog

Booting initramfs image:
------------------------

- Prepare a FTP or TFTP server serving the OpenWrt initramfs image and
  connect the server to a switch port.

- Connect to the console port of the device and enter the extended
  boot menu by typing Ctrl+B when prompted.

- Choose the menu option "<3> Enter Ethernet SubMenu".

- Set network parameters via the option "<5> Modify Ethernet Parameter".
  Enter the FTP/TFTP filename as "Load File Name" ("Target File Name"
  can be left blank, it is not required for booting from RAM). Note that
  the configuration is saved on flash, so it only needs to be done once.

- Select "<1> Download Application Program To SDRAM And Run".

Initial installation:
---------------------

- Boot an initramfs image as described above, then use sysupgrade to
  install OpenWrt permanently. After initial installation, the
  bootloader needs to be configured to load the correct image file

- Enter the extended boot menu again and choose "<4> File Control",
  then select "<2> Set Application File type".

- Enter the number of the file "openwrt-kernel.bin" (should be 1), and
  use the option "<1> +Main" to select it as boot image.

- Choose "<0> Exit To Main Menu" and then "<1> Boot System".

NOTE: The bootloader on these devices can only boot from the VFS
filesystem which normally spans most of the flash. With OpenWrt, only
the first part of the firmware partition contains a valid filesystem,
the rest is used for rootfs. As the bootloader does not know about this,
you must not do any file operations in the bootloader, as this may
corrupt the OpenWrt installation (selecting the boot image is an
exception, as it only stores a flag in the bootloader data, but doesn't
write to the filesystem).

Signed-off-by: Jan Hoffmann <jan@3e8.eu>