[roll third_party/pigweed] pw_uart: Added UART interface

Added `Uart` interface that proves a set of basic methods for
performing UART communication.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/181710

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: d595cc8bc24ce2e..93558a01ab8e995
Roller-URL: https://ci.chromium.org/b/8752303758680063201
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I6a222b4e0f5ca3a32e8676ce53eb1516126c006c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/200672
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index d595cc8..93558a0 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit d595cc8bc24ce2e441e4897542ec56eb7628f600
+Subproject commit 93558a01ab8e99578147806c3c16e3abf276dfaa