powerpc/pseries/mobility: add missing break to default case

update_dt_node() has a switch statement where the default case lacks a
break statement.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201207215200.1785968-9-nathanl@linux.ibm.com