soundwire: add debugfs support

Add base debugfs mechanism for SoundWire bus by creating soundwire
root and master-N and slave-x hierarchy.

Also add SDW Slave SCP, DP0 and DP-N register debug file.

Registers not implemented will print as "XX"

Credits: this patch is based on an earlier internal contribution by
Vinod Koul, Sanyog Kale, Shreyas Nc and Hardik Shah.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Sanyog Kale <sanyog.r.kale@intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20190821185821.12690-2-pierre-louis.bossart@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

include/linux/soundwire/sdw.h

@@ -542,6 +542,7 @@ struct sdw_slave_ops {
  * @bus: Bus handle
  * @ops: Slave callback ops
  * @prop: Slave properties
+ * @debugfs: Slave debugfs
  * @node: node for bus list
  * @port_ready: Port ready completion flag for each Slave port
  * @dev_num: Device Number assigned by Bus
@@ -553,6 +554,9 @@ struct sdw_slave {
 	struct sdw_bus *bus;
 	const struct sdw_slave_ops *ops;
 	struct sdw_slave_prop prop;
+#ifdef CONFIG_DEBUG_FS
+	struct dentry *debugfs;
+#endif
 	struct list_head node;
 	struct completion *port_ready;
 	u16 dev_num;
@@ -729,6 +733,7 @@ struct sdw_master_ops {
  * @m_rt_list: List of Master instance of all stream(s) running on Bus. This
  * is used to compute and program bus bandwidth, clock, frame shape,
  * transport and port parameters
+ * @debugfs: Bus debugfs
  * @defer_msg: Defer message
  * @clk_stop_timeout: Clock stop timeout computed
  * @bank_switch_timeout: Bank switch timeout computed
@@ -748,6 +753,9 @@ struct sdw_bus {
 	struct sdw_bus_params params;
 	struct sdw_master_prop prop;
 	struct list_head m_rt_list;
+#ifdef CONFIG_DEBUG_FS
+	struct dentry *debugfs;
+#endif
 	struct sdw_defer defer_msg;
 	unsigned int clk_stop_timeout;
 	u32 bank_switch_timeout;