iommu/arm-smmu: add option to enable halt/resume of SMMU

Before SMMU is powered down, SMMU needs to be in
idle state prior to power collapse. When 'halt' is
received by SMMU, it ensures that no new requests
enters and all outstanding requests are completed
and generates an acknowledgment for halt request.

Add an option to register a notifier on regulator
so that SMMU can be halted/resumed when regulator
is powered down/up. Some of the targets have built
in provision in hardware to halt SMMU when there
are no on going transactions. We should keep this
option disable for those targets.

Change-Id: Ia2f8a934a9d64daefdacd517eb22f09de5eeb273
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>