[roll third_party/pigweed] pw_persistent_ram: Make persistents mutable

Adds an accessor to pw_persistent that allows non-const access to the
underlying object to enable in-place modification.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/38721

third_party/pigweed Rolled-Commits: f9ae189221f6860..9b085ce0cd03b4c
Roller-URL: https://ci.chromium.org/b/8850742186260608768
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8850742186260608768
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I5165e97e42793351b1ca2b0fd7ee231ac3dc8170
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/40004
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 f9ae189..9b085ce 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit f9ae189221f68600468ee77761a89e0e2c16d010
+Subproject commit 9b085ce0cd03b4cf663b2ba11cc5e1f7e1f82f6f