drivers: gpio: stm32 driver uses a structure to hold the pin configuration

Define a pin structure to carry the pin configuration elements
such as push/pull, opendrain, input/output, high/low.
This structure will give the gpio_flags_t to the gpio_stm32_get_config.
Only the stm32F1x serie does not provide the pin config get function.

Signed-off-by: Francois Ramu <francois.ramu@st.com>