drm/amdgpu: define soc15_ras_field_entry for reuse

The struct soc15_ras_field_entry will be reused by
other IPs, such as mmhub and gc

v2: rename ras_subblock_regs to gc_ras_fields_vg20,
because the future asic maybe have a different table.

Signed-off-by: Dennis Li <dennis.li@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>