mfd: wcd9xxx-core: restructure debugfs interface

Current debugfs interface exposed by wcd9xxx-core is specific to
slimbus slave device as part of WCD CODEC. Plan is to use the
debugfs interface for other purpose as well. Change debugfs interface
to more generic name.

Change-Id: I884639409dcd9bfdaa0fc0f62e670ee091cc7c51
Signed-off-by: Patrick Lai <plai@codeaurora.org>
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>