xen/mm: Introduce mm-types.h

xen/mm: Introduce mm-types.h

The type used for pagetable attributes/permissions is currently unsigned int,
but needs to become architecture dependent as PPC needs unsigned long.

Introduce mm-types.h to house pte_attr_t.

Given the new toolchain baseline, we can use __has_include() now to remove the
need for boilerplate on most architectures.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>