dtb: amd: Add KCS device tree node

Add KCS device node to support IPMI solution on Overdrive
system.

Signed-off-by: Brijesh Singh <brijeshkumar.singh@amd.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Olof Johansson <olof@lixom.net>