Add LED SATA Disk Trigger

Found on the Internet.
Add device tree for ZyXEL NSA325v1 2-Bay NAS

Tested and works:
- All LEDs (SATA1/SATA2/Copy Green/Copy Red/System Green/System Orange/USB Green)
- Buttons (Power, Copy, Reset)
- 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)
- USB 3.0 (xHCI controller)
- 2 disks SATA (With HD 2 power enable GPIO)
- NAND flash

Untested:
- MCU
Enable sata-disk trigger for SATA1 LED by default
Add NSA325 defconfig
Add target "nsa325" to main makefile

Creates an U-Boot kernel image containing the compressed linux kernel and the accompanying NSA 325 OF dtb.