targets/rp2040: Disable pw_rpc mutex since there is no RTOS

Disable the pw_rpc mutex since there is no RTOS. An upcoming Pigweed CL
will add a pw_rpc dependency on pw_thread, unless the mutex is disabled.

Change-Id: Id231deaf6e4adbaaeb08a85dc3c3142e7e639ed3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/126711
Reviewed-by: Alexei Frolov <frolv@google.com>
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/targets/rp2040/BUILD.gn b/targets/rp2040/BUILD.gn
index cada0f2..1dea777 100644
--- a/targets/rp2040/BUILD.gn
+++ b/targets/rp2040/BUILD.gn
@@ -75,6 +75,7 @@
     pw_sync_INTERRUPT_SPIN_LOCK_BACKEND =
         "$dir_pw_sync_baremetal:interrupt_spin_lock"
     pw_sync_MUTEX_BACKEND = "$dir_pw_sync_baremetal:mutex"
+    pw_rpc_CONFIG = "$dir_pw_rpc:disable_global_mutex"
 
     pw_third_party_freertos_CONFIG = "//targets/rp2040:freertos_config"
     pw_third_party_freertos_PORT =