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/include/acpi/acpigen.h

@@ -499,7 +499,7 @@ void acpigen_write_field(const char *name, const struct fieldlist *l, size_t cou
 void acpigen_write_indexfield(const char *idx, const char *data,
 			      struct fieldlist *l, size_t count, uint8_t flags);
 
-int get_cst_entries(acpi_cstate_t **);
+int get_cst_entries(const acpi_cstate_t **);
 
 /*
  * Get element from package into specified destination op: