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/mainboard/libretrend/lt1000/devicetree.cb

@@ -154,11 +154,6 @@ chip soc/intel/skylake
 	# Send an extra VR mailbox command for the PS4 exit issue
 	register "SendVrMbxCmd" = "2"
 
-	# Lock Down
-	register "common_soc_config" = "{
-		.chipset_lockdown = CHIPSET_LOCKDOWN_COREBOOT,
-	}"
-
 	device cpu_cluster 0 on
 		device lapic 0 on end
 	end