drm/amdgpu: add vram_info v2_5 in atomfirmware header

vram_info v2_5 was introduced to support sienna_cichlid

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>