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>