soc/amd: rename common Kconfig and use wildcard for SoC-specific Kconfig

By renaming the AMD SOC common Kconfig file the wildcard to source all
AMD SoC-specific Kconfig files won't match to it and it can be sourced
after all SoC-specific Kconfig files in the sub-directories are sourced.
This change allows adding new SoCs without having to edit the soc/amd
Kconfig file.

Change-Id: Iaaa5aad23eb6364d46b279101f3969db9f182607
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47701
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/soc/amd/Kconfig

@@ -1,6 +1,5 @@
 # Load all chipsets before common
-source "src/soc/amd/picasso/Kconfig"
-source "src/soc/amd/stoneyridge/Kconfig"
+source "src/soc/amd/*/Kconfig"
 
 # Load common defaults last
-source "src/soc/amd/common/Kconfig"
+source "src/soc/amd/common/Kconfig.common"