drm/amdgpu: parse base address from new ip discovery with 64bit ip base address

Truncate the 64bit base address from ip discovery and only store lower 32bit
ip base in reg_offset[].

Bits > 32 follows ASIC specific format, thus just discard them and handle it
within specific ASIC.

By this way reg_offset[] and related helpers can stay unchanged.

v2: make comments more generic

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>