ARM: 8773/1: amba: Export amba_bustype

This patch is provided in the context of allowing the Coresight driver
subsystem to be loaded as modules.  Coresight uses amba_bus in its call
to bus_find_device() in of_coresight_get_endpoint_device() when
searching for a configurable endpoint device.  This patch allows
Coresight to reference amba_bustype when built as a module.

[original LKML submission here: https://lkml.org/lkml/2018/5/9/520]

Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Eric Auger <eric.auger@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Todd Kjos <tkjos@google.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Thierry Reding <treding@nvidia.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Kim Phillips <kim.phillips@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>