ARM: uniphier: make boot_is_swapped() code optional

The planned new SoC does not have SBC (System Bus Controller) block.
Make boot_is_swapped() an optional hook.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/arch/arm/mach-uniphier/sbc/Makefile b/arch/arm/mach-uniphier/sbc/Makefile
index 4efdd41..6c698a3 100644
--- a/arch/arm/mach-uniphier/sbc/Makefile
+++ b/arch/arm/mach-uniphier/sbc/Makefile
@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0+
 
-obj-					+= dummy.o
+obj-y					+= sbc-boot.o
 
 ifndef CONFIG_SPL_BUILD
 obj-y					+= sbc.o