ARM: zynq: use "select" instead of default value in defconfig

All the Zynq boards have switch to Driver Model.
"select DM" is better than default value in each defconfig.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 1c90d17..588e5ee 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -650,6 +650,7 @@
 	bool "Xilinx Zynq Platform"
 	select CPU_V7
 	select SUPPORT_SPL
+	select DM
 
 config TARGET_XILINX_ZYNQMP
 	bool "Support Xilinx ZynqMP Platform"