arm64: trans_pgd: make trans_pgd_map_page generic

kexec is going to use a different allocator, so make
trans_pgd_map_page to accept allocator as an argument, and also
kexec is going to use a different map protection, so also pass
it via argument.

Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
[ made the struct with function pointer const, dropped the comment ]
Signed-off-by: James Morse <james.morse@arm.com>