iommu: arm-smmu: update regulator notification for SMMU

The SMMU halt needs to be accessed before regulator is disabled. So move
notification from REGULATOR_EVENT_DISABLE to REGULATOR_EVENT_PRE_DISABLE.

Regulator framework will notify client on other events too, return
NOTIFY_OK.

Change-Id: Ieed950418d5afc6f5b84b13131ef39964f3168e2
Signed-off-by: Jaydeep Sen <jsen@codeaurora.org>