iommu/arm-smmu: bypass hypervisor and read sync/inval status register

ARM_SMMU_STATS_SYNC_INV_TBU_ACK has been made accessible to HLOS 1, read
it's value directly instead of making an SCM call to retrieve it.

Change-Id: I34f5d18e9a5bdee6857f2398974257a49b1f54fb
Signed-off-by: Chris Goldsworthy <cgoldswo@codeaurora.org>