iommu/arm-smmu: Fix LOG2SIZE setting for 2-level stream tables

STRTAB_BASE_CFG.LOG2SIZE should be set to log2(entries), where entries
is the *total* number of entries in the stream table, not just the first
level.

This patch fixes the register setting, which was previously being set to
the size of the l1 thanks to a multi-use "size" variable.

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