soc/intel: Rename Makefiles from .inc to .mk

The .inc suffix is confusing to various tools as it's not specific to
Makefiles. This means that editors don't recognize the files, and don't
open them with highlighting and any other specific editor functionality.

This issue is also seen in the release notes generation script where
Makefiles get renamed before running cloc.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: Ib479b93b7d0b2e790d0495b6a6b4b4298a515d9a
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80073
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/cannonlake/cnl_memcfg_init.c

@@ -74,7 +74,7 @@ static void meminit_spd_data(FSP_M_CONFIG *mem_cfg, uint8_t mem_slot,
 /*
  * Initialize default memory settings using the spd file specified by
  * spd_index. The spd_index is an index into the SPD_SOURCES array defined
- * in spd/Makefile.inc.
+ * in spd/Makefile.mk.
  */
 static void meminit_cbfs_spd_index(FSP_M_CONFIG *mem_cfg,
 				   int spd_index, uint8_t mem_slot)