[nRF52840] enable I-code cache in nRF52840 by default (#1977)

examples/platforms/nrf52840/platform.c

@@ -34,6 +34,7 @@
 
 #include <openthread/platform/logging.h>
 
+#include <device/nrf.h>
 #include <drivers/clock/nrf_drv_clock.h>
 #include "platform-nrf5.h"
 
@@ -46,6 +47,9 @@ void PlatformInit(int argc, char *argv[])
     (void)argc;
     (void)argv;
 
+    // Enable I-code cache
+    NRF_NVMC->ICACHECNF = NVMC_ICACHECNF_CACHEEN_Enabled;
+
     nrf_drv_clock_init();
 
 #if (OPENTHREAD_CONFIG_ENABLE_DEFAULT_LOG_OUTPUT == 0)