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This patch implements a handler for common SiP calls. A weak
implementation for the SoC-specific handler has been provided
which can be overridden by SoCs to implement any custom SiP
calls.

Change-Id: I45122892a84ea35d7b44be0f35dc15f6bb95193e
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>