drivers: gpio: stm32F1x serie does not provide the pin get config

The stm32F1x serie does not provide the pin_get_config function,
Even if the CONFIG_GPIO_GET_CONFIG is set, the -ENOSYS error code
is returned by the z_impl_gpio_pin_get_config().

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