cpu/intel/speedstep: Constify get_cst_entries()

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>

src/cpu/intel/speedstep/acpi.c

@@ -88,7 +88,7 @@ void generate_cpu_entries(const struct device *device)
 						       CPUs share the same
 						       layout. */
 	int num_cstates;
-	acpi_cstate_t *cstates;
+	const acpi_cstate_t *cstates;
 	sst_table_t pstates;
 	uint8_t coordination;