Add the mips_dsp_rev build variable.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 164b97d26260f3e238834c36ffad1e9e53037f5c..9919a85c234b6760583cc9d059fac598a5f783f9 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -301,6 +301,12 @@ config("compiler") {
             "-Wa,-mips32",
           ]
         }
+
+        if (mips_dsp_rev == 1) {
+          cflags += [ "-mdsp" ]
+        } else if (mips_dsp_rev == 2) {
+          cflags += [ "-mdspr2" ]
+        }
       }
     } else if (cpu_arch == "mips64el") {
       # Don't set the compiler flags for the WebView build. These will come