[roll third_party/pigweed] pw_tls_client: Add fake entropy backend

Add a default fake entropy for demo and test purpose.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/51402

third_party/pigweed Rolled-Commits: fec572b92d89a86..3e6925230aa31d9
Roller-URL: https://ci.chromium.org/b/8842951369684778864
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8842951369684778864
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I64e05b9d80734a66aca03ba0da1210b0bb67b84e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/51822
Reviewed-by: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index fec572b..3e69252 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit fec572b92d89a863d84f35a2cef2199f9872a088
+Subproject commit 3e6925230aa31d9159d56c99c82de923e6aa6764