sunxi: decide the inclusion of SCP by SCP_ADDR existence

There are more Allwinner SoCs that do not have a SCP now.

When there's no SCP_ADDR macro defined, we can assume there's no SCP
available.

Drop the scp part of FIT description when SCP_ADDR does not exist.

Signed-off-by: Icenowy Zheng <icenowy@sipeed.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>