This patch adds IRQ support to STM32 gpios.

The EXTI controller has 16 lines dedicated to GPIOs.
EXTI line n can be connected to only line n of one of the GPIO ports, for
example EXTI0 can be connected to either PA0, or PB0, or PC0...
This port selection is done by specifying the port number into System
Config registers.
Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>