arm64: zynqmp: Enable booting to ATF

U-Boot is now able to boot to ARM Trusted Firmware (ATF). The boot
flow is SPL(EL3) loads ATF and full u-boot and jump to ATF(EL3) which
pass control to full u-boot(EL2). This has been tested on zcu106, so
enable it in this defconfig.

To generate an image that triggers this booting flow, you need to pass
'-O arm-trusted-firmware' to mkimage.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>