xilinx: Enable SMC command for arm64 targets

SMC command is very useful for TF-A testing or issuing commands which are
not covered by any driver. Strongly recommend to disable this command on
any product unless it is required.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/23c77a2cbd083963ca17b84de4108dbb1f28597f.1676450712.git.michal.simek@amd.com