ramips: improve netis WF-2881 support

ramips: improve netis WF-2881 support

This patch does the following:

1. remove u-boot-env partition
   Stock bootloader saves env variables at 0x80000 in flash,
   and there is nothing stored at 0x30000.
   By merging the partition "u-boot-env" with "u-boot",
   the partition table becomes the same as used in stock firmware:
   00000000-00080000: "Bootloader"
   00080000-00100000: "Config"
   00100000-00140000: "Factory"
   00140000-07f80000: "Kernel"

2. fix LAN/WAN MAC addresses and provide label MAC address
   Ethernet MAC addresses are stored in factory partition at:
   0xe000: LAN (label_mac)
   0xe006: WAN (label_mac+1)

3. fix LAN port order
   WF-2881 LAN ports are in reverse order of switch ports.

4. fix WiFi LEDs
   mt76 led nodes are added to make WiFi LEDs work.
   On top of this, mt76 node names are changed to more generic ones,
   and compatible strings are also added.

5. fix pinmux groups
   uart3 and uart2 pins are used as button and led, but jtag pins are not.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>