cc253x: Renamed the P2 ISR

This is mainly a naming convention thing, we want to have 'isr'
as part of the name, instead of 'int'. We also want port_2 instead
of p2 because we already had port_1

See Pull Request #18

cpu/cc253x/dev/p2-intr.c

@@ -34,7 +34,7 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 static struct cc253x_p2_handler *handlers = NULL;
 
 void
-cc253x_p2_int(void) __interrupt(P2INT_VECTOR)
+port_2_isr(void) __interrupt(P2INT_VECTOR)
 {
   struct cc253x_p2_handler *h;
   uint8_t handled = 0;

cpu/cc253x/dev/p2-intr.h

@@ -48,7 +48,7 @@ void cc253x_p2_unregister_handler(struct cc253x_p2_handler *h);
 /* When the ISR is not needed, hide its prototype from the module containing
  * main() to prevent linking it */
 #if PORT_2_ISR_ENABLED
-void cc253x_p2_int(void) __interrupt(P2INT_VECTOR);
+void port_2_isr(void) __interrupt(P2INT_VECTOR);
 #endif
 
 #define cc253x_p2_irq_disable(flag) do { flag = IEN2 & 0x2; IEN2 &= ~0x2; } while(0)