ASoc: simple-mux: add idle-state support

So far the mux changes it state immediately, even when not in use. Allow
overriding this behaviour by specifying an optional idle-state. This
state is used whenever the mux is powered down, only switching to the
selected state on power up. If unspecified it defaults to as-is,
maintaining the previous behaviour.

Signed-off-by: Hendrik v. Raven <h.v.raven@merzmedtech.de>
Link: https://patch.msgid.link/20241114-simple-mux-idle-state-v2-1-a30cb37d2be2@merzmedtech.de


Signed-off-by: Mark Brown <broonie@kernel.org>