drm/amdgpu: add basic support for atomfirmware.h (v3)

This adds basic support for asics that use atomfirmware.h
to define their vbios tables.

v2: rebase
v3: squash in num scratch reg fix

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ken Wang <Qingqing.Wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>