aarch64: mmu: code to split/combine page tables

Two scenarios are possible.

privatize_page_range:

Affected pages are made private if they're not. This means a whole
new page branch starting from the top may be allocated and content
shared with the reference page tables, except for the private range
where content is duplicated.

globalize_page_range:

That's the reverse operation where pages for given range is shared with
the reference page tables and no longer needed pages are freed.

When changing a domain mapping the range needs to be privatized first.

When changing a global mapping the range needs to be globalized last.

This way page table sharing across domains is maximized and memory
usage remains optimal.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>