boot/uboot: add cpu dependency to BR2_TARGET_UBOOT_ZYNQ

Since the Xilinx Zynq SoC family is always based on ARM Cortex-A9 CPUs, the
BR2_TARGET_UBOOT_ZYNQ option should only be available when the BR2_cortex_a9
option is selected.

Signed-off-by: Neal Frager <neal.frager@amd.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

boot/uboot/Config.in

@@ -704,6 +704,7 @@ endif
 config BR2_TARGET_UBOOT_ZYNQ
 	bool "Boot on the Xilinx Zynq SoCs"
 	depends on BR2_arm
+	depends on BR2_cortex_a9
 	help
 	  Enable options specific to the Xilinx Zynq family of SoCs.