roll: third_party/pigweed: pw_digital_io: Add Mock

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/227836
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https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 8436580deec2312..37c87bbae3753e7
Roller-URL: https://ci.chromium.org/b/8740249688960801745
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Id9c1735ef378e7b6550dc88289cec27e15e00656
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/228071
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
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Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 8436580..37c87bb 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 8436580deec23124cefda6cfa55fcdd1d1a86aed
+Subproject commit 37c87bbae3753e7886b41d305398da65983ef0ea