powerpc/numa: Cleanup hot_add_scn_to_nid

This patch reworks the hot_add_scn_to_nid and its supporting functions
to make them easier to understand.  There are no functional changes in
this patch and has been tested on machine with memory represented in the
device tree as memory nodes and in the ibm,dynamic-memory property.

My previous patch that introduced support for hotplug memory add on
systems whose memory was represented by the ibm,dynamic-memory property
of the device tree only left the code more unintelligible.  This
will hopefully makes things easier to understand.

Signed-off-by: Nathan Fontenot <nfont@austin.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>