iommu/arm-smmu: Remove broken big-endian check

The bottom word of the pgd should always be written to the low half of
the TTBR, so we don't need to swap anything for big-endian.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>