kirkwood: add ZyXEL NSA325 device

This patch add ZyXEL NSA325 2-Bay Media Server

The ZyXEL NSA325 device is a Kirkwood based NAS:

- SoC: Marvell 88F6702 1600Mhz
- SDRAM memory: 512MB DDR2 400Mhz
- Gigabit ethernet: Marvell Alaska
- Flash memory: 128MB
- 1 Power button
- 1 Power LED (blue)
- 5 Status LED (green/red)
- 1 Copy/Sync button
- 1 Reset button
- 2 SATA II ports (internal)
- 2 USB 2.0 ports (back)
- 1 USB 3.0 port (front)
- Fan (fixed speed)
- hardware watchdog in a mcu

Basically a bigger, more powerful version of NSA310,
installation is the same as they share the same flash layout.

A notable difference is that there is a hardware watchdog
in a mcu on the board, which is disabled by default in the LEDE u-boot.
The watchdog is also disabled with a GPIO activation through
raw register change when kwbooting or it would reset the board before
the new uboot was transferred.

Signed-off-by: Alberto Bursi <alberto.bursi@outlook.it>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [remove dead code]