sunxi: Mark scp as optional

This binary does not prevent the system from booting. Mark it optional
so that U-Boot can be built without it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/arch/arm/dts/sunxi-u-boot.dtsi b/arch/arm/dts/sunxi-u-boot.dtsi
index 0909a67..e1a9a7f 100644
--- a/arch/arm/dts/sunxi-u-boot.dtsi
+++ b/arch/arm/dts/sunxi-u-boot.dtsi
@@ -90,6 +90,7 @@
 					scp {
 						filename = "scp.bin";
 						missing-msg = "scp-sunxi";
+						optional;
 					};
 				};
 #endif