soc/intel/skl: Constify `soc_get_cstate_map()`

soc/intel/skl: Constify `soc_get_cstate_map()`

This is a follow-up to commit e9f10ff38b which changed the base
signature and all other occurrences.

To make gcc11 happy (which is pickier about these things), let skylake
follow.

Change-Id: I42a629d865baa53640213a03e54e85623a386e35
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58458
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>