Add a soft IH ring implementation similar to the hardware IH1/2.

This can be used if the hardware delegation of interrupts to IH1/2
doesn't work for some reason.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>