[roll third_party/pigweed] pw_thread_freertos: Add a constant for the maximum thread priority

Since FreeRTOS uses a highest priority value is the highest priority
approach, define a constant for the maximum thread priority.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/56543

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 8577f74379924e7..0b6b443601ad4fd
Roller-URL: https://ci.chromium.org/b/8839334915464200704
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8839334915464200704
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I06f758399ddeee142c21c2d59e6a1129bf9bfd23
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/56550
Bot-Commit: 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 8577f74..0b6b443 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 8577f74379924e7a94602cfd12cd0640a61b24da
+Subproject commit 0b6b443601ad4fda0867727622b3afbbd254d3bb