ARM: kirkwood: add support for OpenBlocks A7 platform

The OpenBlocks A7 board is designed and sold by PlatHome, and based on
a Kirkwood 6283 Marvell SoC. It is quite similar to the OpenBlocks A6
already supported in the kernel, with the following main differences:

 - The A6 uses a RTC on I2C, while the A7 uses the internal SoC RTC.

 - The A6 has one Ethernet port, while the A7 has two Ethernet ports

 - The A6 has only one USB port, while the A7 integrates a USB hub,
   which provides two front-side USB port, and an internal USB port as
   well.

 - The A6 has 512 MB of RAM, while the A7 has 1 GB of RAM.

 - Slightly different GPIOs for some functions.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>