bus: introduce an Marvell EBU MBus driver

The Marvell EBU SoCs have a configurable physical address space
layout: the physical ranges of memory used to address PCI(e)
interfaces, NOR flashes, SRAM and various other types of memory are
configurable by software, through a mechanism of so-called 'address
decoding windows'.

This new driver mvebu-mbus consolidates the existing code to address
the configuration of these memory ranges, which is spread into
mach-mvebu, mach-orion5x, mach-mv78xx0, mach-dove and mach-kirkwood.

Following patches convert each Marvell EBU SoC family to use this
driver, therefore removing the old code that was configuring the
address decoding windows.

It is worth mentioning that the MVEBU_MBUS Kconfig option is
intentionally added as a blind option. The new driver implements and
exports the mv_mbus_dram_info() function, which is used by various
Marvell drivers throughout the tree to get access to window
configuration parameters that they require. This function is also
implemented in arch/arm/plat-orion/addr-map.c, which ultimately gets
removed at the end of this patch series. So, in order to preserve
bisectability, we want to ensure that *either* this new driver, *or*
the legacy code in plat-orion/addr-map.c gets compiled in.

By making MVEBU_MBUS a blind option, we are sure that only a platform
that does 'select MVEBU_MBUS' will get this new driver compiled
in. Therefore, throughout the next patches that convert the Marvell
sub-architectures one after the other to this new driver, we add the
'select MVEBU_MBUS' and also ensure to remove plat-orion/addr-map.c
from the build for this specific sub-architecture. This ensures that
bisectability is preserved.

Ealier versions of this driver had a DT binding, but since those were
not yet agreed upon, they were removed. The driver still uses
of_device_id to find the SoC specific details according to the string
passed to mvebu_mbus_init(). The plan is to re-introduce a proper DT
binding as a followup set of patches.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>