Add NCP as a log region option in logging.h (#304)

This commit adds new log region for  NCP (kLogRegionNcp). It also
adds the support for posix platform logging implementation.

examples/platforms/posix/logging.c

@@ -103,6 +103,10 @@ void otPlatLog(otLogLevel aLogLevel, otLogRegion aLogRegion, const char *aFormat
     case kLogRegionMem:
         fprintf(stderr, "MEM  ");
         break;
+
+    case kLogRegionNcp:
+        fprintf(stderr, "NCP  ");
+        break;
     }
 
     va_start(args, aFormat);