powerpc/pseries/mobility: Add pr_debug() for device tree changes

When investigating issues with partition migration or resource
reassignments it is helpful to have a log of which nodes and
properties in the device tree have changed. Use pr_debug() so it's
easy to enable these at runtime with the dynamic debug facility.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190627053044.9238-3-nathanl@linux.ibm.com