arm: mvebu: Add basic Armada 38x support

This patch adds support for the Marvell Armada 38x SoC family.

Supported peripherals are:
- UART
- Ethernet (mvneta)
- I2C
- SPI (including SPI NOR flash)

Tested on Marvell DB-88F6820-GP evaluation board.

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: Kevin Smith <kevin.smith@elecsyscorp.com>
Tested-by: Dirk Eibach <dirk.eibach@gdsys.cc>