amdgpu: Enable KFD on POWER systems

KFD has been verified to function on POWER systems (Talos II / Vega 64).
It should be available as a kernel configuration option on these systems.

Signed-off-by: Timothy Pearson <tpearson@raptorengineering.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>