iommu: arm-smmu: Only modify the ACTLR register if required

Modify the behavior when the exact value of the ACTLR register
is not provided via devicetree. Instead of setting it to zero,
keep the default value.

Change-Id: Ibd56983e57f5bf2c73e4d9368d84ae9aa58c193f
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>