drm/amdgpu: Report firmware versions with sysfs v2

Firmware versions can be found as separate sysfs files at:
/sys/class/drm/cardX/device/fw_version (where X is the card number)
The firmware versions are displayed in hexadecimal.
v2: Moved sysfs files to subfolder

Signed-off-by: Ori Messinger <ori.messinger@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>