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In multi-guest trusty environment, all guest's SMCs will be
forwarded to Trusty. This change only allows 1 guest's SMC
to be forwarded at a time and returns 'busy' status to all
other requests.

Change-Id: I2144467d11e3680e28ec816adeec2766bca114d4
Signed-off-by: Anthony Zhou <anzhou@nvidia.com>
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>