backlight: 88pm860x_bl: Use of_get_child_by_name() instead of refcount hack

of_find_node_by_name() calls of_node_put() on its "from" parameter.
To counter this, pm860x_backlight_dt_init() calls of_node_get() first.

Use of_get_child_by_name() instead to get rid of the refcount hack.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Lee Jones <lee.jones@linaro.org>