soc/amd: factor out common SMI/SCI enums and function prototypes

At least a part or the remaining definitions in the soc-specific smi.h
files are also common, but those have to be verified more closely.

Change-Id: I5a3858e793331a8d2ec262371fa22abac044fd4a
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48217
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>

src/soc/amd/stoneyridge/southbridge.c

@@ -18,6 +18,7 @@
 #include <amdblocks/lpc.h>
 #include <amdblocks/acpi.h>
 #include <amdblocks/smbus.h>
+#include <amdblocks/smi.h>
 #include <soc/southbridge.h>
 #include <soc/smi.h>
 #include <soc/amd_pci_int_defs.h>