arm64: mm: Fix build error with CONFIG_SPARSEMEM_VMEMMAP disabled

This fix the below build error:

arch/arm64/mm/dump.c: In function ‘ptdump_init’:
arch/arm64/mm/dump.c:331:18: error: ‘VMEMMAP_START_NR’ undeclared (first use in this function)
  address_markers[VMEMMAP_START_NR].start_address =
                  ^
arch/arm64/mm/dump.c:331:18: note: each undeclared identifier is reported only once for each
function it appears in
arch/arm64/mm/dump.c:333:18: error: ‘VMEMMAP_END_NR’ undeclared (first use in this function)
  address_markers[VMEMMAP_END_NR].start_address =
                  ^
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Jungseung Lee <js07.lee@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>