ARM: dts: msm: Add flag to identify CP mode for msm8937

A secure video session can be configured in different
modes to acheive the desired level of security. This
string can distinguish for the secure mode for any
given target.

Change-Id: I809be619d0a692eb06744c195ece0fff445a5470
Signed-off-by: Vikash Garodia <vgarodia@codeaurora.org>