tock: Bump to the 1.6 release

Bump to the latest release. This now uses a long lasting OpenTitan boot
ROM to avoid CI failures, see https://github.com/tock/tock/pull/2170.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

tock

diff --git a/tock b/tock
index 9f40864..e8d0a28 160000
--- a/tock
+++ b/tock
@@ -1 +1 @@
-Subproject commit 9f408649ce25f46f0dd49dae7f1a0a9849cae2af
+Subproject commit e8d0a28d86897c91b6747be357abfcfa7e86688f