arm64: mm: Consolidate early page table allocation

The logic for early allocation of page tables is duplicated between
pgd_kernel_pgtable_alloc() and pgd_pgtable_alloc(). Drop the duplication
by calling one from the other and renaming pgd_kernel_pgtable_alloc()
accordingly.

Signed-off-by: Will Deacon <will.deacon@arm.com>