iommu/io-pgtable-arm: Ensure ARM_64_LPAE_S2_TCR_RES1 is unsigned

ARM_64_LPAE_S2_TCR_RES1 is intended to map to bit 31 of the VTCR register,
which is required to be set to 1 by the architecture. Unfortunately, we
accidentally treat this as a signed quantity which means we also set the
upper 32 bits of the VTCR to one, and they are required to be zero.

Treat ARM_64_LPAE_S2_TCR_RES1 as unsigned to avoid the unwanted
sign-extension up to 64 bits.

Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>