pinctrl: st: st_pctl_dt_parse_groups simplify expression

for_each_property_of_node(pins, pp) checks that pp is not NULL.
So there is no need to check it inside the loop.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>