drm/amdgpu/psp: add structure to support PSP SPL

Add support for PSP SPL (Security patch level) table to support
anti-rollback of FW loaded by Trusted OS.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>