soc/intel/common: Use CHIPSET_LOCKDOWN_COREBOOT by default

Since all mainboards use `CHIPSET_LOCKDOWN_COREBOOT`, make it the
default by changing its enum value to 0 and remove its configuration
from all related devicetrees.

If `common_soc_config.chipset_lockdown` is not configured with
something else in the devicetree, then `CHIPSET_LOCKDOWN_COREBOOT`
is used.

Also, add a release note for the upcoming 4.15 release.

Change-Id: I369f01d3da2e901e2fb57f2c83bd07380f3946a6
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56967
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Tim Crawford <tcrawford@system76.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

src/soc/intel/common/block/include/intelblocks/cfg.h

@@ -8,8 +8,8 @@
 #include <intelblocks/mmc.h>
 
 enum {
-	CHIPSET_LOCKDOWN_FSP = 0, /* FSP handles locking per UPDs */
-	CHIPSET_LOCKDOWN_COREBOOT, /* coreboot handles locking */
+	CHIPSET_LOCKDOWN_COREBOOT = 0,	/* coreboot handles locking */
+	CHIPSET_LOCKDOWN_FSP,		/* FSP handles locking per UPDs */
 };
 
 /*