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FF-A specification states that error codes should be typed int32_t.
SPMD's uses uint64_t for return values, which if assigned with a signed
type would have sign extension, and change the size of the return from
32-bit to 64-bit.
Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I288ab2ffec8330a2fe1f21df14e22c34bd83ced3