arm64/mm: don't initialize pgd_cache twice

When PGD_SIZE != PAGE_SIZE, arm64 uses kmem_cache for allocation of PGD
memory. That cache was initialized twice: first through
pgtable_cache_init() alias and then as an override for weak
pgd_cache_init().

Remove the alias from pgtable_cache_init() and keep the only pgd_cache
initialization in pgd_cache_init().

Fixes: caa84136 ("x86/mm: Initialize PGD cache during mm initialization")
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>