drm/amdgpu: Add interface to load SRIOV cap FW

- Add interface to load SRIOV cap FW. If the FW does not
  exist, simply skip this FW loading routine.
  This FW will only be loaded under SRIOV. Other driver
  configuration will not be affected.
  By adding this interface, it will make us easier to
  prepare SRIOV Linux guest driver for different users.

- Update sysfs interface to read cap FW version.

- Refactor PSP FW loading routine under SRIOV to use a
  unified SWITCH statement instead of using IF statement

- Remove redundant amdgpu_sriov_vf() check in FW loading
  routine

Acked-by: Monk Liu <monk.liu@amd.com>
Acked-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Bokun Zhang <Bokun.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>