ARM: dts: kirkwood: Add Zyxel NSA310S board

Zyxel NSA310S is a NAS based on Marvell kirkwood SoC.

Specification:
 - Processor Marvell 88F6702 1 GHz
 - 256MB RAM
 - 128MB NAND
 - 1x GBE LAN port (PHY: Marvell 88E1318)
 - 2x USB 2.0
 - 1x SATA
 - 3x button
 - 7x leds
 - serial on J1 connector (115200 8N1) (GND-NOPIN-RX-TX-VCC)

Tested-by: Tony Dinh <mibodhi@gmail.com>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Acked-by: Adam Baker <linux@baker-net.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>