bus: ti-sysc: Add minimal TI sysc interconnect target driver

We can handle the sysc interconnect target module in a generic way
for many TI SoCs. Initially let's just enable runtime PM with
autosuspend, and probe the children. This can already be used for
idling interconnect target modules that don't have any device driver
available for the child devices.

For now, the "ti,hwmods" custom binding is still required. That will
be eventually deprecated in later patches. And more features will be
added, such as parsing for sysc capabilities so we can continue
removing the legacy platform data.

Cc: Benoît Cousson <bcousson@baylibre.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Nishanth Menon <nm@ti.com>
Cc: Matthijs van Duin <matthijsvanduin@gmail.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Peter Ujfalusi <peter.ujfalusi@ti.com>
Cc: Sakari Ailus <sakari.ailus@iki.fi>
Cc: Tero Kristo <t-kristo@ti.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Tony Lindgren <tony@atomide.com>