ramips: add support for Sitecom WLR-4100 v1 002

ramips: add support for Sitecom WLR-4100 v1 002

Sitecom WLR-4100 v1 002 (marked as X4 N300) is a wireless router
Specification:
SoC: MT7620A
RAM: 64 MB DDR2
Flash: MX25L6405D SPI NOR 8 MB
WIFI: 2.4 GHz integrated
Ethernet: 5x 10/100/1000 Mbps QCA8337
USB: 1x 2.0
LEDS: 2x GPIO controlled, 5x switch
Buttons: 1x GPIO controlled
UART: row of 4 unpopulated holes near USB port, starting count from
      white triangle on PCB:

    VCC 3.3V
    GND
    TX
    RX

    baud: 115200, parity: none, flow control: none

Installation

    Connect to one of LAN (yellow) ethernet ports,
    Open router configuration interface,
    Go to Toolbox > Firmware,
    Browse for OpenWrt factory image with dlf extension and hit Apply,
    Wait few minutes, after the Power LED will stop blinking, the router is
    ready for configuration.

Known issues
Some USB 2.0 devices work at full speed mode 1.1 only

MAC addresses

factory partition only contains one (binary) MAC address in 0x4.
u-boot-env contains four (ascii) MAC addresses, of which two appear
to be valid.

  factory     0x4       **:**:**:**:b9:84  binary
  u-boot-env  ethaddr   **:**:**:**:b9:84  ascii
  u-boot-env  wanaddr   **:**:**:**:b9:85  ascii
  u-boot-env  wlanaddr  00:AA:BB:CC:DD:12  ascii
  u-boot-env  iNICaddr  00:AA:BB:CC:DD:22  ascii

The factory firmware only assigns ethaddr. Thus, we take the
binary value which we can use directly in DTS.

Additional information
OEM firmware shell password is: SitecomSenao
useful for creating backup of original firmware.
There is also another revision of this device (v1 001), based on RT3352 SoC

The nvmem feature (commit 06bb4a5) was introduced in master after the
splitting of the 21.02 branch. It need to be reverted in 21.02..

Signed-off-by: Andrea Poletti <polex73@yahoo.it>
[remove config DT label, convert to nvmem, remove MAC address
 setup from u-boot-env, add MAC address info to commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit de0c380a5f8289839ab970e794a45f0e04a466a3)

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