Add crypto initializer to nrf52840 port (#1246)

examples/platforms/nrf52840/misc.c

@@ -33,6 +33,11 @@
 
 static uint32_t sResetReason;
 
+__WEAK void nrf5CryptoInit(void)
+{
+    // This function is defined as weak so it could be overridden with external implementation.
+}
+
 void nrf5MiscInit(void)
 {
     // Read the reason of last reset.

examples/platforms/nrf52840/platform-nrf5.h

@@ -78,7 +78,7 @@ void nrf5RandomInit(void);
 void nrf5LogInit(void);
 
 /**
- * Function for processing SPI Slave driver.
+ * Initialization of Misc module.
  *
  */
 void nrf5MiscInit(void);
@@ -95,4 +95,10 @@ void nrf5RadioInit(void);
  */
 void nrf5RadioProcess(otInstance *aInstance);
 
+/**
+ * Initialization of hardware crypto engine.
+ *
+ */
+void nrf5CryptoInit(void);
+
 #endif  // PLATFORM_NRF5_H_

examples/platforms/nrf52840/platform.c

@@ -53,6 +53,7 @@ void PlatformInit(int argc, char *argv[])
     nrf5RandomInit();
     nrf5UartInit();
     nrf5MiscInit();
+    nrf5CryptoInit();
     nrf5RadioInit();
 #if (OPENTHREAD_ENABLE_DEFAULT_LOGGING == 0)
     nrf5LogInit();