amdgpu: sync up mullins firmware with 18.20 release

From internal commit:
4e9bb623d83a96a783fd92ad1a4d9b05a3284d7c

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>

WHENCE

@@ -1954,6 +1954,15 @@ File: amdgpu/kabini_sdma.bin
 File: amdgpu/kabini_sdma1.bin
 File: amdgpu/kabini_uvd.bin
 File: amdgpu/kabini_vce.bin
+File: amdgpu/mullins_ce.bin
+File: amdgpu/mullins_me.bin
+File: amdgpu/mullins_mec.bin
+File: amdgpu/mullins_pfp.bin
+File: amdgpu/mullins_rlc.bin
+File: amdgpu/mullins_sdma.bin
+File: amdgpu/mullins_sdma1.bin
+File: amdgpu/mullins_uvd.bin
+File: amdgpu/mullins_vce.bin
 File: amdgpu/topaz_ce.bin
 File: amdgpu/topaz_k_smc.bin
 File: amdgpu/topaz_mc.bin