boards/nrf52840dk: support ieee802.15.4 radio

boards/common/nrf52/Makefile.dep

+include $(RIOTCPU)/nrf52/Makefile.dep
+
 ifneq (,$(filter skald,$(USEMODULE)))
   USEMODULE += nrfble
 endif

boards/common/nrf52/include/cfg_timer_default.h

@@ -36,10 +36,17 @@ static const timer_conf_t timer_config[] = {
         .channels = 3,
         .bitmode  = TIMER_BITMODE_BITMODE_32Bit,
         .irqn     = TIMER1_IRQn
+    },
+    {
+        .dev      = NRF_TIMER2,
+        .channels = 3,
+        .bitmode  = TIMER_BITMODE_BITMODE_08Bit,
+        .irqn     = TIMER2_IRQn
     }
 };
 
 #define TIMER_0_ISR         isr_timer1
+#define TIMER_1_ISR         isr_timer2
 
 #define TIMER_NUMOF         (sizeof(timer_config) / sizeof(timer_config[0]))
 /** @} */

boards/nrf52840dk/Makefile.dep

 include $(RIOTBOARD)/common/nrf52xxxdk/Makefile.dep
+
+ifneq (,$(filter gnrc_netdev_default netdev_default,$(USEMODULE)))
+  ifeq (,$(filter nrfmin,$(USEMODULE)))
+    USEMODULE += nrf802154
+  endif
+endif

boards/nrf52840dk/Makefile.features

 include $(RIOTBOARD)/common/nrf52xxxdk/Makefile.features
+
+# Various other features (if any)
+FEATURES_PROVIDED += radio_nrf802154
+FEATURES_PROVIDED += periph_pwm