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At present, both SDEI_PRIVATE_RESET and SDEI_SHARED_RESET returns
SDEI_PENDING if they fail to unregister an event. The SDEI specification
however requires that the APIs return SDEI_EDENY in these cases. This
patch fixes the return codes for the reset APIs.

Change-Id: Ic14484c91fa8396910387196c256d1ff13d03afd
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>