arch/arm64/mmu: fix page table reference counting

Existing code confused table usage and table reference counts together.
This obviously doesn't work. A table with one reference to it and one
populated PTE is not the same as a table with 2 references to it and
no PTe in use.

So split the two concepts and adjust the code accordingly. A page needs
to have its PTE usage count drop to zero before the last reference is
released. When both counts are 0 then the page is free.

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