ARM: mvebu: add DT support for Seagate Personal Cloud

This patch adds DT support for the Seagate Personal Cloud 1 and 2-Bay.

Here are some information allowing to identify these devices:

Product name                 | Personal Cloud | Personal Cloud 2-Bay
Code name (board/PCB)        | Cumulus        | Cumulus Max
Model name (case sticker)    | SRN21C         | SRN22C
Material desc (product spec) | STCRxxxxxxx    | STCSxxxxxxx

Chipset list:
- SoC Marvell Armada 370 88F6707, CPU @1GHz
- SDRAM memory: 512MB DDR3 667MHz (16-bits bandwidth)
- SPI flash 1MB (Macronix MX25L8006E)
- 1 or 2 SATA internal ports
- 1 Ethernet Gigabit port (PHY Marvell 88E1518)
- 1 USB3 host port (PCIe controller ASM1042)
- 1 USB2 host port (SoC)
- 2 push buttons (power and reset)
- 1 SATA LED (bi-color, white and red)

Note that support for the white SATA LED is missing. A dedicated LED
driver is needed.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>