tock: Bump to the 1.6 release
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 <52ecf00b70a50079ae8d5fa09b9af6904950b63c@wdc.com>

tock/libtock-rs