[radio] otPlatRadioGetNow fallback to otPlatTimeGet (openthread#10646)

This commit makes the default otPlatRadioGetNow implementation fallback
to to otPlatTimeGet so that the default one is a correct implementation
in single chip architecture.

src/core/radio/radio_platform.cpp

@@ -253,7 +253,7 @@ OT_TOOL_WEAK uint64_t otPlatRadioGetNow(otInstance *aInstance)
 {
     OT_UNUSED_VARIABLE(aInstance);
 
-    return UINT64_MAX;
+    return otPlatTimeGet();
 }
 
 OT_TOOL_WEAK uint32_t otPlatRadioGetBusSpeed(otInstance *aInstance)