powerpc/mm: Separate ibm, dynamic-memory data from DT format

We currently have code to parse the dynamic reconfiguration LMB
information from the ibm,dynamic-meory device tree property in
multiple locations; numa.c, prom.c, and pseries/hotplug-memory.c.
In anticipation of adding support for a version 2 of the
ibm,dynamic-memory property this patch aims to separate the device
tree information from the device tree format.

Doing this requires a two step process to avoid a possibly very large
bootmem allocation early in boot. During initial boot, new routines
are provided to walk the device tree property and make a call-back
for each LMB.

The second step (introduced in later patches) will allocate an
array of LMB information that can be used directly without needing
to know the DT format.

This approach provides the benefit of consolidating the device tree
property parsing to a single location and (eventually) providing
a common data structure for retrieving LMB information.

This patch introduces a routine to walk the ibm,dynamic-memory
property in the flattened device tree and updates the prom.c code
to use this to initialize memory.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>