arm64: xilinx: Enable EFI_HTTP_BOOT by default

Enable EFI_HTTP_BOOT to be able to booting OS via http.
In case of that dhcp server is not providing dns server IP set it up via
setenv dnsip <ip addr>.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/b78a7d8b0100c724f657c0997b273e073cf31a14.1706093917.git.michal.simek@amd.com