MIPS: Move pgd_alloc() out of header

pgd_alloc() references init_mm which is not exported to modules. In
order for KVM to be able to use pgd_alloc() to allocate GVA page tables,
move pgd_alloc() into a new pgtable.c file and export it to modules.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org