realtek: support EnGenius EWS2910P v3

realtek: support EnGenius EWS2910P v3

Add support for V3 of the Engenius EWS2910P PoE switch. Like its v1
brother, This is an RTL8380 based switch with two SFP slots, and PoE
802.3af one every RJ-45 port.

Unlike its older brother, the max budget is 55W instead of 61.6 W.
Investigation into the communication protocol with the PoE controller
is ongoing, though it appears the vendor firmware configures the PSE
with a per-port budget of 30.0W.

Specifications:
---------------
* SoC:       Realtek RTL8380M
* Flash:     32 MiB SPI flash Macronix MX25L25635E
* RAM:       256 MiB (As reported by bootloader)
* Ethernet:  8x 10/100/1000 Mbps with PoE
             2x SFP slots
* Buttons:   1 "Reset" button on front panel
             1 "LED mode: button on front panel
             1 "On/Off" Toggle switch on the back
* Power:     48V-54V DC barrel jack
* UART:      1 serial header (JP1) with populated 2.54mm pitch header
             Labeled GRTV for ground, rx, tx, and 3.3V respectively
* PoE:       1 STM ST32... microcontroller (U15)
             1 RTL8238B PSE controller
Works:
------
  - (8) RJ-45 ethernet ports
  - Switch functions
  - LEDs and buttons

Not yet enabled:
----------------
  - Power-over-Ethernet (requires realtek-poe support for RTL8232B)

Install via web interface:
-------------------------

The factory firmware will accept and flash the initramfs image. It is
recommended to flash to "Partition 0". Flashing to "Partition 1" is
not supported at this point.

The factory web GUI will show the following warning:

 " Warning: The firmware version is v0.00.00-c0.0.00
     The firmware image you are uploading is older than the current
     firmware of the switch. The device will reset back to default
     settings. Are you sure you want to proceed?"

This is expected when flashing OpenWrt. After the initramfs image
boots, flash the -sysupgrade using either the commandline or LuCI.

Install via serial console/tftp:
--------------------------------

See commit 2cfaab4549f5 ("realtek: add support for EnGenius EWS2910P").

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/15217
Signed-off-by: Robert Marko <robimarko@gmail.com>