soc/intel: Constify soc_get_cstate_map()

Return a read-only pointer from the `soc_get_cstate_map()` function.
Also, constify the actual data where applicable.

Change-Id: I7d46f1e373971c789eaf1eb582e9aa2d3f661785
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58392
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

src/soc/intel/cannonlake/acpi.c

@@ -111,7 +111,7 @@ static int cstate_set_s0ix[] = {
 	C_STATE_C10
 };
 
-acpi_cstate_t *soc_get_cstate_map(size_t *entries)
+const acpi_cstate_t *soc_get_cstate_map(size_t *entries)
 {
 	static acpi_cstate_t map[MAX(ARRAY_SIZE(cstate_set_s0ix),
 				ARRAY_SIZE(cstate_set_non_s0ix))];