soc: Add Kconfig for each soc vendor

Allows explicit ordering for vendors that share a common configuration
that must be sourced last.

The issue is that chips in soc/{amd,intel}/[ab].* will be able to
override defaults set in this file, but Kconfig files that get sourced
later (soc/amd/[d-z].*) will NOT be able to override these defaults.

Note: intel and amd soc chips now need to be added manually to the new
Kconfig file

BUG=b:62235314
TEST=make lint-stable

Change-Id: Ida82ef184712e092aec1381a47aa1b54b74ed6b6
Signed-off-by: Chris Ching <chingcodes@google.com>
Reviewed-on: https://review.coreboot.org/22123
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/broadcom/Kconfig

@@ -0,0 +1,2 @@
+# Load all chipsets
+source "src/soc/broadcom/*/Kconfig"