kirkwood: add support for ipTIME NAS1

kirkwood: add support for ipTIME NAS1

ipTIME NAS1 is a 1-bay NAS, based on Marvell Kirkwood SoC.

Specifications:
* SoC: 88F6281
* RAM: 256 MiB
* Flash: SPI NOR 16 MiB
* SATA: 1x 3Gb/s
* Ethernet: 1x 1GbE
* USB: 1x 2.0
* Fan: 2 speed level
* UART: JP1 (115200 8N1)
  * Pinout: [3V3] (TXD) (RXD) (GND)

Notes:
* There are several variants of the model name: "NAS-I", "NASI", "NAS1".
  Here "NAS1" is adopted for consistent naming scheme.
* The reset button is also a USB copy button in stock FW,
  but in this patch the former is the only default behavior.

Installation via web interface:
1.  Flash sysupgrade image through the stock web interface.

Revert to stock firmware:
1.  Perform sysupgrade with stock image.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>