drivers: soc: Add mutex lock in audio_notifier_late_init

Audio_notifier_late_init registers clients dependent on PDR to
correct service if registration to PDR on init fails.
There is a possible race condition when audio_notifier_late_init
and audio_notifier_register is called at same time. This may
cause manipulation of the same lists and critical data at the
same time. Add mutex lock to protect critical data.

CRs-Fixed: 2016927
Change-Id: Iec5190a93b390d46f47fd67fd79b1b83d7fe4b29
Signed-off-by: Rohit Kumar <rohitkr@codeaurora.org>