arm: mvebu: Add Armada A38x DB-88F6820-GP board support

This patch adds support for the Marvell DB-88F6820-GP Armada A38x
evaluation board.

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

Please note that this board support right now only supports the
main U-Boot. Without the bin_hdr integration (DDR training etc). This
will be added in a few days / weeks to complete this board port. But
till then this U-Boot version can be run on the target via the
original Marvell U-Boot via this command:

tftpboot 4000000 db-88f6820-gp/u-boot.bin;go 4000000

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