dt-bindings: bus: Add documentation for Tegra210 ACONNECT

Add binding documentation for the Tegra ACONNECT bus that is part of the
Audio Processing Engine (APE) on Tegra210. The ACONNECT bus is used to
access devices within the APE subsystem. The APE is located in a
separate power domain and so accesses made to the ACONNECT require the
power domain to be enabled as well as some platform specific clocks.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>