arm64: hibernate: add trans_pgd public functions

trans_pgd_create_copy() and trans_pgd_map_page() are going to be
the basis for new shared code that handles page tables for cases
which are between kernels: kexec, and hibernate.

Note: Eventually, get_safe_page() will be moved into a function pointer
passed via argument, but for now keep it as is.

Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: James Morse <james.morse@arm.com>
[will: Keep these functions static until kexec needs them]
Signed-off-by: Will Deacon <will@kernel.org>