realtek/rtl839x: Edgecore ECS4100-12PH support

realtek/rtl839x: Edgecore ECS4100-12PH support

Add support for the Edgecore ECS4100-12PH, an 8-port 802.3bt PoE Gigabit
Ethernet switch with 2 combo RJ45/SFP and 2 SFP ports.

Hardware:
* SoC: RTL8393M
* RAM: 256MiB
* Flash: 32MiB SPI-NOR
* Ethernet:
  * 8x GbE RJ45 PoE (external RTL8218B)
  * 2x GbE RJ45 / SFP combo (external RTL8214FC)
  * 2x SFP (external RTL8214FC)
* Console: RJ45 RS232 port on front panel
* PoE: Nuvoton M0516 + 2x Broadcom BCM59121 PSE

Installation via bootloader:
* open serial console (baud rate 115200)
* interrupt boot process by pressing any key during boot
* boot the OpenWrt initramfs:
  # rtk network on
  # tftpboot 0x8f000000 /tftpboot/openwrt-realtek-rtl839x-edgecore_ecs4100-12ph-initramfs-kernel.bin
  # bootm
* copy openwrt-realtek-rtl839x-edgecore_ecs4100-12ph-squashfs-sysupgrade.bin
  to /tmp and use sysupgrade to install it:
  # sysupgrade /tmp/openwrt-realtek-rtl839x-edgecore_ecs4100-12ph-squashfs-sysupgrade.bin

Even though U-Boot claims the switch is based on the RTL8392M SoC, my
device is based on the RTL8393M SoC. I have confirmed this by removing
the heatsink, and the Linux kernel agrees with this. Therefore the DTS
has the rtl8393_ prefix.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>