EDAC: Document HW_EVENT_ERR_DEFERRED type

Add a description of the HW_EVENT_ERR_DEFERRED type that wasn't included
with commit d12a969e

 ("EDAC, amd64: Add Deferred Error type").

Signed-off-by: Yazen Ghannam <Yazen.Ghannam@amd.com>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>