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Previously, ZynqMP IPI in ATF is only for ZynqMP PM,
This patch is to have a ZynqMP IPI implementation to handle
both ZynqMP PM IPI requirement and IPI mailbox service requirement
which will be introduced next.

We control IPI agents registers access but not IPI buffers access in
this implementation. Each IPI mailbox user will directly access the
IPI buffers.
Signed-off-by: Wendy Liang <jliang@xilinx.com>