[NUCLEOs] Improvement of gpio_irq hal

Now it is possible to have multiple interrupt pins on EXTI vectors.
Example:
EXTI0 -> PA_0 or PB_0 or PC_0, ... only 1 pin
EXTI5_9 -> PA_5 and PA_6 and PB_7 and PC_8 and PC_9: up to 5 pins
EXTI10_15: PA_10 and PB_11 and PB_12, ... : up to 6 pins
Merge pull request #720 from bcostm/master

Targets: NUCLEOs - Improvement of gpio_irq hal