[NXP][platform][common] Fix the building error if "defines" exists in the invoker

Signed-off-by: Chin-Ran Lo <nxf57500@lsv051039.swis.nl-cdc01.nxp.com>

Author: Chin-Ran Lo

third_party/nxp/rt_sdk/rw61x/rw61x.gni

@@ -205,7 +205,7 @@ template("rw61x_sdk_drivers") {
     libs = []
 
     if (defined(invoker.defines)) {
-      defines += invoker.defines
+      defines = invoker.defines
     } else {
       defines = []
     }