intel_th: Only create useful device nodes

Right now, the driver will create a device node for each output port,
with the intent to provide read access to that port's data. However,
only the memory ports are readable this way (msc0, msc1). Other output
ports don't need device nodes, so remove them.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>