interrupt_controller: Fix clang compilation issues in intc_exti_stm32.c

Clang reports 'initializer element is not a compile-time constant' when
trying to initialize structure with elements of constant array. The
compiler is wrong in this case because the elements of the array are
known at compile time, so there are no obstacles to initialize structure
using it.

The problem was fixed by removing 'range' array and using
DT_PROP_BY_IDX() to get values directly from DT.

Signed-off-by: Patryk Duda <pdk@semihalf.com>