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>
diff --git a/src/soc/intel/common/block/acpi/acpi.c b/src/soc/intel/common/block/acpi/acpi.c
index f144a6d..af7212e 100644
--- a/src/soc/intel/common/block/acpi/acpi.c
+++ b/src/soc/intel/common/block/acpi/acpi.c
@@ -248,7 +248,7 @@
 
 static void generate_c_state_entries(void)
 {
-	acpi_cstate_t *c_state_map;
+	const acpi_cstate_t *c_state_map;
 	size_t entries;
 
 	c_state_map = soc_get_cstate_map(&entries);