ARM: mvebu: add DT support for Seagate NAS 2 and 4-Bay

This patch adds DT support for the Seagate NAS 2 and 4-Bay.

Here are some information allowing to identify these devices:

Product name                 | Seagate NAS 2-Bay | Seagate NAS 4-Bay
Code name (board/PCB)        | Dart 2-Bay        | Dart 4-Bay
Model name (case sticker)    | SRPD20            | SRPD40
Material desc (product spec) | STCTxxxxxxx       | STCUxxxxxxx

Chipset list (common):
- SoC Marvell Armada 370 88F6707, CPU @1.2GHz
- SDRAM memory: 512MB DDR3 600MHz (16-bits bandwidth)
- NAND flash 256MB, 8-bits (Micron MT29F2G08AAB or Hinyx H27U2G8F2CTR-BC)
- 2 SATA II ports (SoC)
- 1 Ethernet Gigabit ports (PHY Marvell 88E1518)
- 2 USB3 host ports (PCIe controller ASM1042)
- GPIO fan (4 speeds)
- External I2C RTC (MCP7940NT)
- 3 push buttons (power, backup and reset)
- 2 SATA LEDs (bi-color, blue and red)
- 1 power LED (bi-color, blue and red)

Only on 4-Bay models:
- 2 extra SATA III ports (PCIe AHCI controller Marvell 88SE9170)
- 1 extra Ethernet Gigabit ports (PHY Marvell 88E1518)
- I2C GPIO expander (PCA9554A)
- 2 extra SATA LEDs (bi-color, blue and red)

Note that support for the white SATA LEDs associated with HDDs 0 and 1
is missing. A dedicated LED driver is needed.

Signed-off-by: Vincent Donnefort <vdonnefort@gmail.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>