drm/amdgpu/swsmu: add SMU mailbox registers in SMU context

So we can eventaully use them in the common smu code for
accessing the SMU mailboxes without needing a lot of
per asic logic in the common code.

Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/pm/swsmu/inc/amdgpu_smu.h

@@ -563,6 +563,10 @@ struct smu_context
 	struct stb_context stb_context;
 
 	struct firmware pptable_firmware;
+
+	u32 param_reg;
+	u32 msg_reg;
+	u32 resp_reg;
 };
 
 struct i2c_adapter;