pinctrl: stm32: check node status before new gpio bank registering

Register a new GPIO bank only if GPIO bank node is enabled. This patch also
adds checks on ranges which are defined only if a bank is registered.

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>