drm/amdgpu: new macro for determining 2ND_USB20PORT support

Used for determining 2ND_USB20PORT support from firmware_capability.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>