mfd: wcd9xxx-core: add power_state debugfs entry

In order to assist test scenarios which need audio CODEC to be off,
add debugfs interface to allow force shutdown/startup of CODEC. The design
is to leverage SLIMBUS device down and device up notification assuming
such test would only be exercised on CODEC in SLIMBUS mode. In addition,
power state command should be issued only when there is absolutely no
audio use case running.

Change-Id: I7c22694e632e4576e3e4a996da1077fa97369cb6
Signed-off-by: Patrick Lai <plai@codeaurora.org>
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>