MAINTAINERS: Adding Maintainers for AMD Seattle Device Tree

Adding maintainers for AMD Seattle device tree.

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