msm: pm: Notify secure code of L2 power mode from last core only

Notify secure code L2 low power mode only if the current core is the
last core down. In certain scenarios, its possible that Linux and
secure code have different views of what a last core and could result in
secure code not invalidating a cache when the L2 was placed in a
non-retention mode.

Consider a two core scenario where Core 1 is power collapsed and Core 0
is the last core down setting L2 to be invalidated. Before Core 0 makes
the scm call, its possible that is interrupted to process a secure
interrupt. If the Core 1 wakes up in this time and decides to vote a
different L2 mode, this mode wouldn't be consistent with what Core 0
passes down to secure code when resuming from interrupt handling. In
these cases, the secure code could end up incorrectly invalidating the L2
on warmboot.

Also, the secure code and Linux should have a consistent view of what
last core down is to flush and invalidate L2. To ensure this, acquire a
remote (hardware) spinlock to serialize the last portion of the
power-collapse sequence. The remote spinlock will be released by the
secure code after it acquires its own lock to serialize the power
collapse sequence.

Change-Id: Ib5e2f360fddce558d22ce094dd7755c395c45a8c
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>