pinctrl: stm32: fix bank io port number

In case the exti line is not in line with the bank number (that is the case
when there is an hole between two banks, for example GPIOK and then GPIOZ),
use "st,bank-ioport" DT property to get the right exti line.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>