bus: Add support for Tegra ACONNECT

Add a bus driver for the Tegra ACONNECT which is used to interface to
various devices within the Audio Processing Engine (APE). The purpose
of the bus driver is to register child devices that are accessed via
the ACONNECT bus and through the device parent child relationship,
ensure that the appropriate power domain and clocks are enabled for
the ACONNECT when any of the child devices are active. Hence, the
ACONNECT driver simply enables runtime-pm for the ACONNECT device
so that when a child device is resumed, it will enable the power-domain
and clocks associated with the ACONNECT.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>