SDEI: Fix return value of reset calls
SDEI: Fix return value of reset calls

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 <5b289de972918ffd274229d00560f010009baf9e@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware