[PATCH] powerpc numa: Get rid of "numa domain" terminology

Since we effectively treat the domain ids given to us by firmare as
logical node ids, make this explicit (basically s/numa_domain/nid/).

No functional changes, only variable and function names are modified.

Signed-off-by: Nathan Lynch <nathanl@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>