arm: mvebu: Add basic support for Armada 375 eval board db-88f6720

This patch adds basic support for the Marvell A375 eval board. Tested
are the following interfaces:
- I2C
- SPI
- SPI NOR
- Ethernet (mvpp2), port 0 & 1

Currently the A375 SerDes and DDR3 init code is not intergrated. So
the SPL U-Boot is not fully functional.

Right now, this A375 mainline U-Boot can only be used by chainloading
it via the original Marvell U-Boot. This can be done via this
command:

=> tftpboot 00800000 a375/u-boot-dtb.bin;go 00800000

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>