lantiq/xrx200: add support for the Fritzbox 7360v2

lantiq/xrx200: add support for the Fritzbox 7360v2

This commit adds support for the Fritzbox 7360v2

CPU: VR9 500MHz Cores: 2
RAM: 128 MB
NOR-Flash: 32 MB
WLAN: AR9287-BL1A

DECT is not working.

Thanks Sebastian Ortwein for adding 7360SL.
The dts file is derived from avm_fritz7360sl.dts.

Firmware can be flashed with this method:

1.) Set your client IP to 192.168.178.2
2.) Power on your your Fritzbox and connect to 192.168.178.1
    via ftp in the first 5 seconds.
3.) login with adam2/adam2
4.) type into the ftp prompt:

passive
binary
debug 1
quote MEDIA FLSH // (not FLASH)
put openwrt-lantiq-xrx200-avm_fritz7360v2-squashfs-sysupgrade.bin mtd1
// using the correct location for the squashfs-sysupgrade-firmware.bin

5.) wait till red light flashing turns off.
6.) type: exit

Run tested with kernel 4.19 and 5.4 on Fritzbox 7360 V2.

Issue:
Ethernet speed is slow, (iperf between a Xiaomi mir3g
and this router results in <80Mbits throughput
with a wired cable when using the gbit ports.)

Signed-off-by: Yushi Nishida <kyro2man@gmx.net>