arm: mvebu: Add basic support for the Marvell Armada 375 SoC

This patch adds basic support for the Armada 375. Please note that
currently the SerDes and DDR3 init code for the A375 is not
included / enabled. This will be done in a later, follow-up patch.

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>