Make the `get_cst_entries()` function provide a read-only pointer. Also,
constify the actual data where applicable.
Change-Id: Ib22b3e37b086a95af770465a45222e9b84202e54
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58393
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>