iommu/io-pgtable-arm: Move some definitions to a header

Extract some of the most generic TCR defines, so they can be reused by
the page table sharing code.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200918101852.582559-6-jean-philippe@linaro.org
Signed-off-by: Will Deacon <will@kernel.org>