lantiq: add support for AVM FRITZ!Box 7362 SL

lantiq: add support for AVM FRITZ!Box 7362 SL

Hardware:
- SoC: Lantiq VRX288
- RAM: Winbond W971GG6JB 1 Gb (128 MiB)
- Flash:
  - SPI: 8 Mb (1 MiB) for bootloader and tffs
  - NAND: 1 Gb (128 MiB) for OS
- xDSL: Lantiq VRX208
- WLAN: Atheros AR9381
- DECT: Dialog Semiconductors SC14441

Everything except FXS/DECT works
(no drivers for AVM's FXS implementation with SC14441).

Installation via FTP:
1. Use scripts/flashing/eva_ramboot.py to send initramfs-kernel.bin
to the device when powering on.
Standard AVM procedures with finding the correct IP address and
the right moment to open FTP apply here (approx. 4 seconds on 7362SL).
IMPORTANT: set lzma compression in ramdisk options, bootloader stalls
when receiving uncompressed images.

2. Transfer sysupgrade.bin image with scp to /tmp directory
and run sysupgrade

3. First boot might take a bit longer if linux_fs_start was set to 1,
in that case the device will reboot twice, first time it will fail to load
second kernel (overwritten by ubifs), set linux_fs_start to 0 and reboot.

OpenWrt uses the entire NAND flash. Kernel uses 4 MiB and rootfs uses
the rest of 124 MiB, overwriting everything related to FRITZ!OS - both
OS images, config and answering machine/media server data.
To return to FRITZ!OS, use AVM's recovery image.

Signed-off-by: Danijel Tudek <danijel.tudek@gmail.com>