iommu/arm-smmu: Optimized IOMMUS property parsing

Previously, parsing of the iommus property nodes was done at every probe
call, making it time-consuming. This change makes the property parsing
outside the probe call at arm_smmu_init and stores the parsing info in a
data structure and later the probe calls can utilize this data structure
to perform the parsing in a time-efficient manner.

Change-Id: Ifd5558b94c134158c5cf15ca6b9e325ba45c55ba
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>