aarch64: mmu: Add initial support for memory domains

Introduce the basic support code for memory domains. To each domain
is associated a top page table which is a copy of the global kernel
one. When a partition is added, corresponding memory range is made
private before its mapping is adjusted.

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