arm64: mm: Fix column alignment for UXN in kernel_page_tables

UXN is the only individual PTE bit other than the PTE_ATTRINDX_MASK ones
which doesn't have both a set and a clear value provided, meaning that the
columns in the table won't all be aligned. The PTE_ATTRINDX_MASK values
are all both mutually exclusive and longer so are listed last to make a
single final column for those values. Ensure everything is aligned by
providing a clear value for UXN.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>