ARM: dts: kirkwood: Add Check Point L-50 board

This patch adds dts for the Check Point L-50 from 600/1100 series
routers.

Specification:
-CPU: Marvell Kirkwood 88F6821 1200MHz
-RAM: 512MB
-Flash: NAND 512MB
-WiFi: mPCIe card based on Atheros AR9287 b/g/n
-WAN: 1 Gigabit Port (Marvell 88E1116R PHY)
-LAN: 9 Gigabit Ports (2x Marvell 88E6171(5+4))
-USB: 2x USB2.0
-Express card slot
-SD card slot
-Serial console: RJ-45 115200 8n1
-Unsupported DSL

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>