soundwire: keep track of Masters in a stream

soundwire: keep track of Masters in a stream

A multi link bankswitch can be done if the hardware supports and
the stream is handled by multiple Master(s).

This preparatory patch adds support to track m_rt in a stream.
Modifying m_rt_count and usage is added as part of the next patch.

Signed-off-by: Shreyas NC <shreyas.nc@intel.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>