lantiq: add Netgear DM200 support

lantiq: add Netgear DM200 support

Netgear DM200 is an inexpensive VDSL modem:

    CPU: VRX220 MIPS 34Kc 5.6 @ 500Mhz
    RAM: 64MiB
    Flash: 8MiB
    Ethernet: 1x100M
    DSL: VDSL2+, ADSL2+
    reset button, 3x bi-color LEDs

Serial port is 115200 baud, on the 4 pin header; pins from the bottom to
top are GND, RX, TX.

To upgrade from the vendor firmware, upload factory.img

The DM200 bootloader supports flashing over TFTP; hold the reset button
while powering on the device, and wait for the power light to start
flashing green before releasing. The device is now listening on
192.168.0.1/24, and can be sent a factory.img or a netgear image with a
TFTP put.

Once the image is loaded, it will be written to the flash, and the
device will reboot; this will take a few minutes.

Thanks to Edward O'Callaghan and Baptiste Jonglez, who implemented their
own ports for this device and provided valuable feedback.

Signed-off-by: Thomas Nixon <tom@tomn.co.uk>