power: qcom: apm: acquire spinlock during APM mode transitions

In order to guarantee the APM switch sequence is not preempted as
well as to provide synchronization in the event the APM switch mode
APIs are called concurrently, introduce a per-APM controller spinlock
which must be acquired and released during the VDD_APCC to VDD_MX
and VDD_MX to VDD_APCC switch sequences.

Change-Id: I630498c512c58cf07754553b75357375158a544c
Signed-off-by: Osvaldo Banuelos <osvaldob@codeaurora.org>