amd/seattle: Initial revision of AMD Seattle support

amd/seattle: Initial revision of AMD Seattle support

This patch add initial (minimal) platform support for AMD Seattle,
which mainly just define the matching ID, and specify system_off,
and system_reset mechanism.

Initially, the firmware only support a subset of PSCI-0.2 functions,
system-off and system-reset. The mechanism for bring up auxiliary processors
is still using spin-table.

Signed-off-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>