misc: Add support for the Arm Versatile Express config bus

misc: Add support for the Arm Versatile Express config bus

Add support for the Arm Versatile Express config bus that is
being used for exposing various subsystems via a generic
configuration bus. This driver adds support for generating
transactions on this configuration bus and can be used by
other drivers to abstract the communication with the actual
function providers.

Signed-off-by: Liviu Dudau <liviu.dudau@foss.arm.com>
Reviewed-by: Heiko Schocher <hs@denx.de>