platform/x86/amd: pmc: Add defines for STB events

Currently `amd-pmc` has two events, but just adds one to the first to
distinguish the second.  Add a clear definition what these events mean.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://lore.kernel.org/r/20220829162953.5947-3-mario.limonciello@amd.com


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>