mm/memblock: add a name for memblock flags enumeration

Since kernel-doc does not like anonymous enums the name is required for
adding documentation. While on it, I've also updated all the function
declarations to use 'enum memblock_flags' instead of unsigned long.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>