soc/amd/mendocino: Add STB Spill-to-DRAM enum

This is the enum value to initialize the Smart Trace Buffer's
Spill-to-DRAM feature.  More information on how this is used is
available in the STB Linux kernel driver.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: Iab2e5fb121902959ddd0e7c8cca930a327b69291
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68686
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>

src/soc/amd/mendocino/include/soc/smu.h

@@ -12,6 +12,7 @@
 
 enum smu_message_id {
 	SMC_MSG_S3ENTRY = 0x0b,
+	SMC_MSG_SET_S2D_ADDR = 0x53,
 };
 
 /*