ARM: uniphier: select SPL_DM rather than default in defconfig

Now UniPhier SoCs highly depend on Driver Model for SPL, too.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index a99ae28..5f3af77 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -817,6 +817,7 @@
 	select SPL
 	select OF_CONTROL
 	select DM
+	select SPL_DM
 	select DM_SERIAL
 	select DM_I2C
 	help