ARM: Kirkwood: DT board setup for LaPlug

This patch adds DT board setup for the LaCie NAS LaPlug.

Chipset list:
- CPU MARVELL 88FR131 800Mhz
- SDRAM memory: 128MB DDR2-800 400Mhz
- 1 Ethernet Gigabit port (PHY MARVELL 88E1318)
- 1 Mini PCI-Express port
- 1 NAND 512 MB
- 1 push button
- 2 LEDs (red and blue)
- 4 USB Ports

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
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Changes v1 -> v2:
  - Rename the board from ns-plug to laplug, changes everywhere
    accordingly
  - Retab the files
  - Use defines for GPIO active state and key input's name.
  - Remove #*-cells in gpio-keys as this is not a bus
  - Add marvell,88e1318s in the compatible line
  - Create kirkwood-6192.dtsi (TO BE MODIFIED FOR PINCTRL)