ASoC: rsnd: add debugfs support

Current rsnd supports #define DEBUG, but it is not helpful
if issue happen after 4-5 hours.
This patch adds debugfs support for it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Message-Id: <87y2c0lwyn.wl-kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>