Add the mips_dsp_rev build variable.

build/config/mips.gni

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # MIPS arch variant.
 if (cpu_arch == "mipsel") {
   declare_args() {
     mips_arch_variant = "r1"
   }
 } else if (cpu_arch == "mips64el") {
   if (is_android) {
     declare_args() {
       mips_arch_variant = "r6"
     }
   } else {
     declare_args() {
       mips_arch_variant = "r2"
     }
   }
 }
+
+# MIPS DSP ASE revision. Possible values are:
+#   0: unavailable
+#   1: revision 1
+#   2: revision 2
+# Default: 0.
+if (cpu_arch == "mipsel") {
+  declare_args() {
+    mips_dsp_rev = 0
+  }
+}