memblock: let memblock_type_name know about physmem type

Since commit 70210ed9 ("mm/memblock: add physical memory list") the
memblock structure knows about a physical memory list.

memblock_type_name() should return "physmem" instead of "unknown" if the
name of the physmem memblock_type is being asked for.

Link: http://lkml.kernel.org/r/20170120123456.46508-2-heiko.carstens@de.ibm.com


Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Philipp Hachtmann <phacht@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>