zynq: zybo: Move CONFIG_DEFAULT_DEVICE_TREE to defconfig

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Move CONFIG_DEFAULT_DEVICE_TREE from board config
directly to defconfig.

This patch removes compilation warning:
include/configs/zynq_zybo.h:24:0: warning: "CONFIG_DEFAULT_DEVICE_TREE"
redefined [enabled by default]
 #define CONFIG_DEFAULT_DEVICE_TREE zynq-zybo

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

configs/zynq_zybo_defconfig

@@ -2,3 +2,5 @@ CONFIG_SPL=y
 +S:CONFIG_ARM=y
 +S:CONFIG_ZYNQ=y
 +S:CONFIG_TARGET_ZYNQ_ZYBO=y
+CONFIG_OF_CONTROL=y
+CONFIG_DEFAULT_DEVICE_TREE="zynq-zybo"

include/configs/zynq_zybo.h

@@ -21,7 +21,6 @@
 
 #define CONFIG_ZYNQ_SDHCI0
 #define CONFIG_ZYNQ_BOOT_FREEBSD
-#define CONFIG_DEFAULT_DEVICE_TREE	zynq-zybo
 
 /* Define ZYBO PS Clock Frequency to 50MHz */
 #define CONFIG_ZYNQ_PS_CLK_FREQ	50000000UL