drivers: intc: stm32: make stm32_exti_line_t opaque

This commit makes the contents of the stm32_exti_line_t data type opaque to
the EXTI GPIO interrupt controller API users. The GPIO driver is updated
to comply with this API change.

N.B.: while some assertions are removed as part of this commit, they were
broken since forever anyways, so nothing of value is lost.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>