powerpc/pseries/dlpar: Use helper function for_each_child_of_node()

for_each_child_of_node can help to iterate through the device_node,
and we don't need to use while loop. No functional change with this
conversion.

Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240822085430.25753-3-zhangzekun11@huawei.com