soc: qcom: scm: Add LMH specific mutex lock

It is required on some hardware that the Limits Management
secure API be executed in mutual exclusion with certain
other hardware-accessing drivers. Provide a mutex to achieve
this.

Change-Id: Ie0db516e335c29b79e959a0a6e6fb76231af6bf4
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>