arm: mvebu: Add common mbus functions to use on Marvell SoCs

These mbus functions are ported from Barebox. The Barebox version is
ported from Linux. These functions will be first used by the upcoming
Armada XP support. Later other Marvell SoC's will be adopted to use
these functions as well (Kirkwood, Orion).

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: Luka Perkov <luka@openwrt.org>