arm64: mm: fold init_pgd() into __create_pgd_mapping()

The routine __create_pgd_mapping() does nothing except calling init_pgd(),
which has no other callers. So fold the latter into the former. Also, drop
a comment that has gone stale.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>