thunderbolt: Add additional USB4 port operations for retimer access

When accessing retimers when there is no cable connected we are going to
need additional USB4 port operations. First the port needs to be put
into offline mode, and then the sideband channel transactions must be
enabled on the SBTX line. This adds support for these operations.

Signed-off-by: Rajmohan Mani <rajmohan.mani@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>