cpu/atmega328p: external interrupt refactor

cpu/atmega328p/include/periph_cpu.h

@@ -41,6 +41,14 @@ enum {
     PORT_D = 3        /**< port D */
 };
 
+/**
+ * @brief   Available external interrupt pins on the ATmega328p family
+ *
+ * In order of their interrupt number.
+ */
+#define CPU_ATMEGA_EXT_INTS    { GPIO_PIN(PORT_D, 2), \
+                                 GPIO_PIN(PORT_D, 3) }
+
 /**
  * @name   Defines for the I2C interface
  * @{