iommu/arm-smmu: Enlarge STRTAB_L1_SZ_SHIFT to support larger sidsize

Because we will choose the minimum value between STRTAB_L1_SZ_SHIFT and
IDR1.SIDSIZE, so enlarge STRTAB_L1_SZ_SHIFT will not impact the platforms
whose IDR1.SIDSIZE is smaller than old STRTAB_L1_SZ_SHIFT value.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>