drm/amd: Add missing firmware declaration for PSP v15.0.0

drm/amd: Add missing firmware declaration for PSP v15.0.0

PSP v15.0.0 needs both TOC and TA firmware. Without the declaration
it won't get included in initramfs and leads to following failure:

```
Direct firmware load for amdgpu/psp_15_0_0_ta.bin failed with error -2
early_init of IP block <psp> failed -19
Fatal error during GPU init
```

Fixes: 9b24f63d825e7 ("drm/amdgpu: Enable support for PSP 15_0_0")
Reviewed-by: Pratik Vishwakarma <Pratik.Vishwakarma@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org