iommu/arm-smmu: Clear global and context bank fault status registers

After reset these registers have unknown values.
This might cause problems when evaluating SMMU_GFSR and/or SMMU_CB_FSR
in handlers for combined interrupts.

Signed-off-by: Andreas Herrmann <andreas.herrmann@calxeda.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>