Implemented for SGPRs for GFX v8 initially.

(v2) cleanup minor whitespace and remove sanity check and
     addressing is in dwords not bytes
Signed-off-by: NTom St Denis <tom.stdenis@amd.com>
Acked-by: NEdward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>