boot/uboot: use BR2_TARGET_XILINX_FIRMWARE_ARCH_SUPPORTS

Migrate the BR2_TARGET_UBOOT_ZYNQMP dependency to the new
BR2_TARGET_XILINX_FIRMWARE_ARCH_SUPPORTS symbol. Also, add a cpu dependency
on BR2_cortex_a53 since the zynqmp platform is based on the ARM Cortex-A53
and the BR2_TARGET_UBOOT_ZYNQMP should not be available for other CPUs with
aarch64 architecture.

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

@@ -575,7 +575,8 @@ config BR2_TARGET_UBOOT_INITIAL_ENV
 
 config BR2_TARGET_UBOOT_ZYNQMP
 	bool "Boot on the Xilinx ZynqMP SoCs"
-	depends on BR2_aarch64
+	depends on BR2_TARGET_XILINX_FIRMWARE_ARCH_SUPPORTS
+	depends on BR2_cortex_a53
 	help
 	  Enable options specific to the Xilinx ZynqMP family of SoCs.