power: qcom: apm: provide mutual exclusion with LMH register accesses

Acquire the LMH scm mutex when performing the VDD_APCC to VDD_MX
or VDD_MX to VDD_APCC APM switch sequences to guarantee mutual
exclusion between APM mode switches and LMH register accesses.

Change-Id: I1541f0fdb79115991b7ee9cf6e98a5aa7ee7511e
Signed-off-by: Osvaldo Banuelos <osvaldob@codeaurora.org>