drm/amdgpu: Report generic instead of unknown boot time errors

Change the DMESG reporting of unknown errors to "Boot Controller
Generic Error" to align with the RAS SPEC and provide more clarity
to customers.

Signed-off-by: Xiang Liu <xiang.liu@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>