nbio handles misc bus io operations. Handle
differences between different nbio bus versions.

v2: switch checks from RAVEN to APU (Alex)
    squash in raven rev id fetch
    squash in fix uninitalized hdp flush reg index for raven
v3: add some missed RAVEN to APU checks (Alex)
Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>