spapr.c: use g_auto* with 'nodename' in CPU DT functions

spapr.c: use g_auto* with 'nodename' in CPU DT functions

Next patch will use the 'nodename' string in spapr_core_dt_populate()
after the point it's being freed today.

Instead of moving 'g_free(nodename)' around, let's do a QoL change in
both CPU DT functions where 'nodename' is being freed, and use
g_autofree to avoid the 'g_free()' call altogether.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20210120232305.241521-2-danielhb413@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>