genpd: ti: Use for_each_node_with_property() simplify code logic

Let's convert to use for_each_node_with_property() to iterate through the
DT nodes, as to simplify the code logic a bit. No intended functional
change.

Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>