arm: kirkwood: Move some SoC files into new arch/arm/mvebu-common

By moving some kirkwood files into a Marvell common directory, those files
can be used by other Marvell platforms as well. The name mvebu is taken
from the Linux kernel source tree. It has been chosen there to represent
the SoC's from the Marvell EBU (Engineering Business Unit). Those SoC's
currently are:

Armada 370/375/XP, Dove, mv78xx0, Kirkwood, Orion5x

This will be used by the upcoming Armada XP (MV78460) platform support.

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: Luka Perkov <luka@openwrt.org>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>