arm64: mm: allow passing a pgdir to alloc_init_*

To allow us to initialise pgdirs which are fixmapped, allow explicitly
passing a pgdir rather than an mm. A new __create_pgd_mapping function
is added for this, with existing __create_mapping callers migrated to
this.

The mm argument was previously only used at the top level. Now that it
is redundant at all levels, it is removed. To indicate its new found
similarity to alloc_init_{pud,pmd,pte}, __create_mapping is renamed to
init_pgd.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Cc: Laura Abbott <labbott@fedoraproject.org>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>