Add device tree for ZyXEL NSA325 2-Bay Power Plus Media Server

Tested and works:
- All LEDs (SATA1/SATA2/Copy Green/Red, System Green/Orange, USB Green)
- Network (Booting with NFS as root file system works)
- Shutdown using "power off" GPIO
- Serial interface
- RTC (pcf8563)
- USB 2.0 (with "usb power enable" GPIO)

Untested:
- SATA
- NAND
- Buttons (Power, Copy, Reset)
- USB 3.0 (xHCI controller)
- MCU